That's just a snapshot. What NixOS allows you to create configuration that will deploy your OS configured the way you like, possibly post it on places like GitHub deeply a new machine confused the exact same way.
BTRFS and ZFS filesystems offer lightweight snapshots. So you can save the state of the filesystem and restore it. It is often integrated with the package manager and a snapshot is involved before you make change.
NixOS works differently. You have a configuration file, and each time you make change to it NixOS rebuilds itself to its specification from scratch (you might assume it would be a lengthy process, but because of caching only things that are rebuilt are things that you are changing).
This means that things like for example squeezing from KDE to Gnome or X11 to Wayland aren't scary to try and you can easily revert things back, your home directory won't be touched.
Also those things aren't exclusive you can use BTRFS and ZFS on NixOS to and enjoy their benefits.
Yeah, you can if you plan well enough (typically. What I'm trying to illustrate is that this works by taking a snapshot of the disk in time. It's like keeping a working copy of your system on your disk to be able to revert to.
While with NixOS you work with a "recipe" how your system is supposed to be configured. It is much lighter. It is declarative, you change the recipe and get what you described, you change configuration and all packages which you did not mention and are not used by anything are gone. If you update your system you can use the same configuration on it
The thing is that using can still get BTRFS or ZFS and use it to have snapshots too (for example your home directory)
Nah man, 3 months ago i had fedora 38 btrfs, timeshift refused to work because subvolumes wasn't done, but i installed everything in auto gui mode, i did them by the manual after installation, timeshift started working just fine, a week further update to fedora 39 came, i updated, everything broke because of subvolumes, i loaded fedora recovery mode from grub, tried to roll back with timeshift btrfs, it rolled back to 38 but everything was still broke, and more over, whole ssd with this installation became locked, had to recover data from completely locked up ssd, in the middle of the process it locked even further, so i couldn't even copy some files when disk was connected as external
NixOS ended up disappointing me a fair bit. I just tried it recently and the KDE support seems very rough so far, or at least I couldn't find good answers to how to configure it and theme it.
One of the main draw of NixOs is the reproducibility of builds, meaning that redoing the build will provide the exact same output each time, so Nix encourages you to make configuration changes through the package manager. I've mostly overcome my theming woes with home-manager now, but this comment was speaking to a little wrinkle I had when I was trying to learn and take advantage of the OS's features as best I could.
The main configuration handles configuration of the system, home manager project was created to bring similar functionality for the user home directory. That's where the name comes from.
Home manager also works great when using Nix on other systems to manage for files, for example on OS X.