FAQ/nixpkgs
From NixOS Wiki
Question: When I do an import <nixpkgs> {}, which channel or path gets imported.
Answer:
Use nix repl '<nixpkgs>' on shell and then enter path in the repl to know what nixpkgs is currently being used.
Nix tools use $NIX_PATH to decide which path to use in the import call. The first entry in that variable is typically <userhome>/.nix-defexpr/channels . If a user has explicitly added a channel named nixpkgs, then that channel is used in the import <nixpkgs> call. Otherwise, the nixpkgs= named entry is used for the import <nixpkgs> call.
Some tools don't fully respect $NIX_PATH and instead search ~/.nix-defexpr so there might be some unexpected behavior.