You assume here that people only work on the area of the codebase which they are most familiar with. I think a sign of a healthy team is when people are contributing to other parts of the code and not getting stuck in a rut where each area of code only has one maintainer/gatekeeper.

Also, for less experienced members, reviewing the changes of more experienced people is a great way to learn if you properly sit down and understand the change (why it’s being done, why it’s being done this way) rather than just skimming through. In addition to catching mistakes, this is useful to check that code is actually comprehensible to people other than the author and a good time to request more/better comments if necessary.