Oh, I was confused, thanks a lot.

And, indeed, reading every file from top to bottom is very alien to me as a junior.

I would just try to get to the file I thought the change I needed was made and start trying and error. Definitely not checking the core files, much less creating a mental model of the architecture (the very concept of architecture would be alien to me then).

I would do get lost in irrelevant details (because I thought they were relevant), while completely missing the details that did matter.