Maybe it's because I'm a less experienced programmer, but I find I have a lot of trouble understanding source code if I'm not trying to find or change something specific. I can't read it like a book, I have to experiment with it.
Your ability to navigate a codebase, to able to see how its components relate to each other and to the effects the software has on the external world, is the perhaps the most valuable skill you can develop as a programmer. If you also have strong communication skills, you will go far.
Sometimes a question will come up during a meeting, nobody knows the answer, so I’ll just pull up the source and because it’s already handy and since I know the structure I can find the answer in a matter of seconds.
The entire set is good and I occasionally go back and revisit them.
It's tedious, slow work, but you can use note-taking, the REPL, tests (reading, if any, but also writing them), diagrams or any other tool for creating mental models until you manage it.