The fact that people open PRs because they are working on a new part of the code means that there's knowledge silos in the team that could be easily solved with code reviews.
Most codebases that I‘ve worked on are too big to know everything. Often it isn‘t even required to build a feature. I think it‘s unavoidable to touch new parts of a growing codebase and you are probably not a good reviewer in the first place if you don‘t know the part of the codebase.
It might help to see some patterns.