How could I create a custom linux distro/iso without using special tools like respin and archiso?

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It was about as easy as configuring my own NixOS system.

Probably true but also potentially misleading

Making a distro requires constant packaging, updating and bugfixing. I dont think a single person should do that

Tadaaam: LinuxMint.

without using special tools like respin and archiso

What do you exactly mean with this?

Can we suggest any package/tool that you have to install?
Did you intend to convey a solution that's independent of an existing distro?
Do you want the custom distro/iso to only do its thing until installation? Or are you perhaps interested in something more declarative that can continue to exist and be (one of) the primary means to config your system?
Could you explain to us how your envisioned solution looks like?

Sorry for asking these questions, but it was either this or a very very long post satisfying all kinds of different criteria. Thanks you in advance for answering any of the questions!

I've been struggling to make a bootable iso. I did Linux from scratch and I wanted to boot it in virtual box. I found a sparse amount of info about mkisofs/genisofimage but I couldn't actually get a successful boot after following a few tutorials.

I have to imagine there are more modern tools for something like this but I didn't have any luck googling.

Sorry to hijack but it sounds like you might have an answer I need. I just want a way to put together an iso with a bootloader that works in virtual machines. (I'm good with 32bit grub but I'd work with uefi too).
- Would you be fine with some tooling that enables one to make their own custom iso from an existing distro? This path still allows for a substantial amount of freedom, though it's not a blank slate by any stretch of the imagination. But it makes up for it with how relatively easy and painless it can be.
  
  Or would you instead like to get into the nitty-gritty of things and want all the freedom you'd want? This increased freedom does come with a substantial cost in convenience and labour.
  
  Pick your poison :P . I'll be waiting ;) .

What do you need to accomplish by creating a new distro? Because it can vary depending on the edits you do on the base distro

Hijacking for my own idea: what if I wanted to make my own updated version of say, hiren's Linux environment that included tools for data recovery and drive administration among other things. I would love to basically build my own custom recovery environment preloaded with all my favorite tools.

Tbf, with a few hours of googling, I could probably figure this out for myself so don't go through a bunch of trouble unless you want to document for others. I'm mainly posting because I saw OP and wondered if something easy already existed?
- This sounds like NixOS could make this fairly easy.
- Well if you want to make a completely independent distro you could try linux from scratch and package the distro as an ISO with the packages you want and maybe try to add a package manager if you can. However i doubt that would be worth it as linux from scratch takes a lot of time and efforts to build and will rarely be better than just building a custom arch or debian ISO

Unless you're trying to build custom AMIs for a cloud environment (packer is the answer for this job) then it sounds like a nightmare of a project to maintain long term. You'd be much better off using config management and a more or less vanilla base distro.

Do you want to distribute it (in mass)? Otherwise i don't see the point you can run a script that you use every time you install the OS

If you want a kali-based live environment, you can follow this tutorial: https://www.kali.org/docs/development/live-build-a-custom-kali-iso/ You can configure a lot of options and most of the building process is just downloading packages.