Nix is a big rabbit hole. You can use it as a normal package manager, but there's a lot more.
You can also use nix-shell to temporarily install packages, it drops you into a shell with your package and that package is gone when you exit the shell. You can also declare a shell using a shell.nix file with environment variables and packages you want (including specific versions) and enter that shell by running nix-shell.
You can also declare your full user environment using nix home manager. Using a home.nix file you can list packages you need and configure everything. That also makes it easy to backup your user environment by just copying the home.nix file.
This is similar to Fairphone's situation. For the Fairphone 5 they just use an IoT chip with long term support from Qualcomm enabling them to give at least 5 years of feature updates and 8 years of security patches.