Packages may be the wrong word. “Files” is probably the term I should have used.
In this post Using NixOS in an isolated environment - #23 by ejpcmac @ejpcmac discusses a process for pulling down a copy of the cache server from the channel list, then checking for missing packages which are required for building the current NixOS system that don’t exist in that cache and pulling them from the local nix-store. I don’t know what those files are. I think most of them are Latex files, but I would need a better answer than that.