Depending on the contract, publishers might have the ability to get a copy of the source code, but they certainly are not going to be monkeying around with it.

Doing a full build from scratch of a game is a complicated thing. The build systems at a game studios are quite extensive.

Even if you were given access to the version control repositories for a game I think it would probably take you quite a long time to be able to successfully export and build everything from scratch.