You can create regression tests that prove that the code does what it does today - which helps you understand it, provides a readable description of what it does and protects against regression bugs in the future.

Here's a relevant trick I frequently use for this kind of work with Python: https://simonwillison.net/2020/Feb/11/cheating-at-unit-tests...

First of all you’re not going to be all by yourself. Ask previous developers or peers if there are any. Ask them to help you navigate the codebase and find good candidates.

On the opposite scenario, where you have to take ownership of a large codebase and there’s no one around that can guide you through it, if every time you learn something about it you turn it into a test case you will do your future self a favor and at the same time also tracing your progress.