Paraphrasing: if I don't write the codebase [but someone else does], I don't write the tests, and other team members are modifying it, what even is considered a messy codebase?

I actually don't see a connection between the mechanism used to create the code and the code messiness. Things like code repetition, incorrect level of abstractions, tests testing only tests themselves, using too smart optimizations for things that don't matter, .. These all can happen in both person- and machine -authored code.

I would be surprised if a professional software developer has never seen at least some aspects of messy codebase in most any large project. The difference can be whether this messiness ever managed, or just piled on.