short question by an aspiring user

Just as a note, I believe you still need to tick the "Enable Steam Play for all titles" in Steam settings to allow it to be used with non-verified games.

There is a GUI, but I prefer the terminal:

sudo apt update
sudo apt install steam

"Update" fetches the latest package information, and "install steam" does exactly what you think it does :)

In the program menu ("Start" menu), search for Software Manager. It's basically an app store.

It's called Software Manager in Mint. It's in the start menu. It's basically like an app store, but without the surveillance and ads. Just type steam in the search field and click install. Really couldn't be easier.

For sure. I think there's a happy medium for those who might not go for the Gentoo approach. (I'm a veteran of Gentoo as well, by the way. :) )

The extreme opposite of that is probably fearing to touch anything once the system is up and running. There are certainly Windows users like that. IT folks have one or two in their families who regularly try to rope them into doing free tech support. ("Sorry, Aunt Debbie, but I haven't touched any version of Windows since XP. No, Aunt Debbie, I don't 'build computers' for my job. That's a different department. No, Aunt Debbie, I don't know how to recover deleted emails in Hotmail. I've never used Hotmail.") I wouldn't want folks to fall into a habit of being afraid of their Linux system.

And of course, the Gentoo or LFS approach is way too far on the other end of the spectrum for some.

But I definitely wasn't advocating that OP take the "break all the things and learn how to recompile your Kernel to enable debugging with GDB so you can figure out why such-and-such USB device isn't working correctly." (Unless of course OP wants to do that. In which case, knock yourself out, OP!)

I used OpenSUSE before Gentoo. I'm glad I did. It got me some basic bearings in the Linux ecosystem in a gentle way that didn't make me want to give up and reinstall Windows. I switched to Gentoo basically when I started to realize how limiting relying on the OpenSUSE guis for installing and configuring things was. (I could tell there was a hidden layer of stuff going on behind those guis. And shying away from the deep lore was keeping me from doing things I could otherwise do.)

But even if I thought a particular person had a strong likelihood of taking the Gentoo approach at some point, I'd probably recommend something like Mint until they themselves wanted to dig deeper. And if that never happened, that's fine too.

And, let's be honest. There's a chance that Mint could break as well even if OP isn't doing reckless things solely for the sake of learning. (I'd say the same about Windows for that matter.) At that point, OP's options are 1) figure out how to fix it and fix it or 2) wipe evrything and reinstall from scratch. Either way, something will have been learned in the process.

So, to OP, don't feel pressured to do all the deep lore stuff unless/until you find yourself wanting to. But also you might be better off if you aren't so scared to try to do things that you don't try to customize your system for your needs in even very simple ways.

And again, good luck!

I just meant with the C: comment that OP shouldn't expect it to still be called C: after he's wiped Windows and is running Linux.

As for the oversimplification comment:

First off, C: (or D:, E: etc) doesn't refer to a disk in Windows. It refers to a partition. So it's entirely possible (and not terribly uncommon) to have a single disk with both a C: and a D: on it.

It's very typical for a Linux installation process to (by default, if you don't tell it to do something else) make separate partitions on a single disk for / and /home. (Plus there's usually an extra EFI boot partition on most modern desktop/laptop systems. And a swap partition.) In such a case, you couldn't really describe where "the disk" (that was formerly called C: on Windows) was mounted in the mindset of conflating "partition" with "disk". What was previously "the disk" C: (again, C: isn't a disk, it's a partition, but Windows makes it easy to conflate the two) is now split in two (or three or more) and mounted not just on / but also on /home (and maybe on /boot as well, and maybe one partition isn't mounted on the main abstract root filesystem).

/ and /home aren't really even partitions (let alone disks). They're mount points in the slightly more abstract root filesystem.

The most obvious software representation on a typical Linux system of the main internal disk in that machine would probably be something like /dev/sda or /dev/nvme0. The partitions would likely be something like /dev/sda1//dev/sda2/etc or /dev/nvme0p1//dev/nvme0p2/etc. Also, the "filesystem" on the partition is arguably a subtlely a distinct concept from the block device that is the partition. And where that filesystem is mounted is yet another distinct concept. (Another subtlety I haven't mentioned is the distinction between the device in the /dev/ directory/filesystem and the kernel representation of the device with the device major/minor numbers.)

A typical Windows install kindof conflates a lot of these probably a lot more so than Linux does. But I didn't want to be like "akshuly things are a lot more complicated than that and you have to understand a bunch of Linux kernel internals to understand all the ways in which you're wrong so you can install the holy 'Guh-noo Plus Linux'." All that is stuff that OP will learn by installing and using Linux. And if OP's going with Mint, it's probably not necessary to really understand all of that before starting the install process.

And technically OP doesn't really need to understand that the main disk won't be called C: after switching to Linux. Probably. (I don't think I've ever installed Mint. So I don't know for sure, but from what I've heard about it, I'd be surprised if the installation process had much of a learning curve.) But I told OP anyway. So there. :D

If "D" is physically on the same hard drive, then you'll probably want to back it up before installing. Technically, you can manage to do it without screwing everything up, but I would not trust myself to. It's always a good idea to have backups anyway.

Also, user files typically reside on C by default and it takes some effort to put them on a different drive. Things like Downloads, Documents, Pictures, etc. so it's worth checking that before wiping as well.

Additionally, you'll probably want to format your "D" drive to a Linux native filesystem (eventually, after you back it up, because formatting results in data loss). While Linux does support NTFS quite well, it's not perfect, and your data would probably be safer on ext4 or f2fs (depending on if you have HDDs or SSDs) (or zfs or btrfs is you're into COW filesystems).

In Linux, you have all of your files mounted to a single "drive" called /. Everything is below /, which is called the "root" of your filesystem.

Typically, user data is stored in "/home" and this resides in the same directory structure as the rest of your OS, but on most systems it's on a different filesystem or even on a different drive entirely. This is because in Linux it is routine to put a "D" drive just in a folder. On my computer, I have several of these mount points defined, so the different types of data don't get mixed around, and I don't have to worry about downloading too much bullshit affecting my computer's updates.

Hope this helps.

@OmnipotentEntity makes a good point. Most (and definitely older) laptops have 1 drive. Which would mean your C and D 'drives' are actually 2 partitions on 1 physical hard drive. This is fine, but you need to be extra cautious when installing Linux. Many linux installers push you to the easiest choice and select 'wipe whole disk and install linux', which in your case would possibly lead to inadvertently wiping the D partition too.

You might want to pay extra attention to this during the installation, when selecting which disk to use for installation. Make sure you only let the installer delete the C partition (which will probably not be named as such, so be ready to find another way to identify the correct partition (maybe by its size?)), and let the installer use the free space that gives to create linux partitions it needs there (next to the D partition).

NB: Still in Windows, you may also want to check whether or not your D drive is encrypted with Bitlocker, as that is a Windows-type encryption and cant be unlocked without a recovery key (aside from it not being practical to use Bitlocker encryption in combination with linux (or NTFS for that sake, as OmnipotentEntity also already mentioned)). If so, you might want to decrypt the D partition so you can still access it from Linux (while it is of course better to have encryption enabled, it may be a temporary convenience).

Edit: Solid choice of Linux Mint btw. It's been a while since I've used it, but in my memory (also as a starting Linux user) it made the right things easy. If your laptop is quite old, and Cinnamon (also solid) doesn't feel quite snappy enough, you could give XFCE a try. It's less polished (some say ugly 😆) out-of-the-box, but also less resource hungry (Cinnamon and XFCE are both Desktop Environments (DE's). On Linux you can have multiple DE's installed side-by-side; and then make a choice which DE you want to use when you login)

That's a good MO to have. I was talking about the partitions that Windows does on its own, depending on the version, of course, and the ones that the manufacturer does. For example, Lenovo has a recovery partition with a (supposedly) untouched installer for the Lenovo bloatwared OOTB Windows installation. Since you never plan on using that laptop for that version of windows again, you can just merge all partitions on that drive, and let the Mint installer use the whole drive for its purpose.

That's even easier than I imagined, thanks

Thank you for your extensive answer, I feel confident now to do the "leap of faith"

You could but it often updates much later than others, and is way too white everywhere