With flakes, you can also lock the project or your system to an exact commit of the nixpkgs repo, meaning you get the same versions until you update the lock file. It's like a npm or cargo lock file, but for the whole system.

The nix packages define how to build and configure it, so the build part is like Gentoo. It has a powerful cache setup, so you rarely need to really build anything. Need a custom kernel though? Define your patches in your config and it works exactly the same until you update it.