I find it helpful to think of the nix client and nix daemon to be separate things.
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The nix client is responsible for evaluating the nix expression language and produces build recipes (.drv files) and fixed-output files. The fixed-output files are when file paths are being used for example. Or any of the builtins.fetch* methods. It then talks to the nix daemon to add those to the store and build them.
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The nix daemon is responsible for the whole
/nixfolder. It has a RPC protocol to receive fixed-output files and .drv files. It can be asked to build derivations… Some of those derivations have a determined fixed output and are also implemented as fetchers.
Even in single-user mode, the nix client and daemon are just in the same process.