I think Michael A. Jackson summed this up best:

Rules of Optimization:

Rule 1: Don’t do it.

Rule 2 (for experts only): Don’t do it yet.

sometimes you’re mentally exhausted after finishing one or more major features.

This is the time to take stock and ease off a little. Perhaps do some small tasks, sometimes known as “chores”, which are less mentally taxing, but nevertheless worthwhile. Maybe take time to pay off some technical debt.

Remember the dictum “If you think education is expensive, try ignorance”.

You have to find a way in. Sometimes trying the code out as a user gives you a better idea of what it’s all about. Sometimes you need to read the docs or tests to get an idea of the externals. Eventually, you can start looking at the source code and building up a mental model of how it all fits together to achieve its purpose.

Feeling overwhelmed is often due to trying to rush the process of gaining understanding. Nobody comes to a new codebase and instantly understands it.

You may have favourite programming languages that will influence your choice of language and tooling, regardless of what would actually make the job in hand easier to complete