It's unlike most other Linux systems I've ever used. The central idea is declarative structures. So your entire operating system, all of the apps and systems services and other custom things you need, are declarative in one place and hopefully easy to activate.
So instead of flatpak install, you add the program to a 'text file' and rebuild your system.
Everything is using the nix package manager which has been around since long before the distro nixos.
That's only the start of the rabbit hole, but if anything sounds interesting i encourage you to check it out!
It checks everything that has changed since last build and it changes only what is needed, I'd say 30 seconds or less normally. When updateing, you can end up updating all of the packages on your system at once, which of course is dependent on your network speed / sometimes compile time, but for me it can take around 10-15 minutes in that case. On a slow network, with a laptop CPU.
But you get atomic rollbacks so any breaking changes can be rolled back just by rebooting and selecting a different build.