>You start by rejecting those PRs, saying "write more maintainable code, not quick hacks".

How do you go about that when for example, my previous employer just allowed any software developer to commit to any branch, and there was never any code review happening?

Set rules on the repo. For example, no one on my project is allowed to check in code unless it's on a PR that's been reviewed and approved by at least two people. Attempting to push to /main will just fail otherwise.