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.
Some assholes gave congresscritters a bunch of money to get their businesses a cozy monopoly and special treatment. They were seeing AI as a profit center they could corner the market on thanks to govt-industry collusion. Looks like ginormous data centers and export controlled GPU cards may not be as essential to AI research as thought, and now the emergency is their stock is tanking.