All of the above. NixOS is actually fairly well suited to a full desktop OS. They package-up basically everything you would expect. You can even pull in Steam

All of the above. NixOS is actually fairly well suited to a full desktop OS. They package-up basically everything you would expect. You can even pull in Steam its proton environment without much hassle. It is also perfectly workable as simple server OS running on bare metal or a VM. One can use it on other OSes as a way to create fully deterministic environments for developing. (Like the python virtualenv concept, but for any language, pulling any arbitary tools you might need). Almost like a docker container, but without isolating the filesystem. Using just nix tool not as part of NixOS, can be an underwhelming experience, as the default does not quite give the fully declarative environment setup you might want. For example, under the default you can imperatively request packages to be added to the nix store. But if you chose to use the "correct" versions of commands, you can get the intended result. Alternatively, using it in conjunction with tools like direnv (which enables you to configure your shell to ask nix for a specific environment when you cd into it, or alternatively when you run a specific command within it). This environment can have exactly the set of tools you need for developing the project that lives in this folder, even if the rest of your OS has a different and incompatible version of the compilers installed, or whatever.