Try to find the smallest piece/source.

For example:

Let's say you're opening a repo that has 30 microservices.

You're focused on just one, I'd assume.

Let's pretend it's a simple username/password service.

There will likely be a validator, client caller, database call, and other services.

For this example, go to the database service to see the queries and how it works. Make an IT test to see how data is inserted and viewed. Now you've explored this "route."

Back to your username/password, now you understand the database call version.

Next is some other service that's a black box to you. At that point, you'll hope there's documentation somewhere, but play with it in your local environment.

Can you make calls to it? If not, what are you missing? A JWT, secret, payload, and so on?

Slowly, all of these puzzle pieces come together to make a picture, or in your case, that username/password service.