In NixOS you have only one central package store `/nix/store`, so all packages need to be installed there. This is necessary, because the software is patched to use that path. IMO patching it this way is a bit hacky, when there a better options.

If overlayfs is used, the software would not require patching, so you could have multiple package stores if you like. Maybe a package store in `~/.nix/store` as well. This would allow with a immutable rootfs (e.g. immutable `/nix/store`) to still install packages per user, or even per project a different package store.