Packages in nix are in the store directory, each package in a dir named after the package hash. So you can have 15 versions of firefox installed, for instance, and the different versions go in different folders with different hashnames.
When it’s time to set up a user env, their specific version of firefox is (conceptually) symlinked into the users profile. When that user executes firefox it gets one out of the 15 versions. Another user may get a different one.
Anyway, the package store is off limits to users, and a real bad idea to modify for root too.
you mean they got the date wrong on a release? that’s not a security issue