Here is another take how you could get yourself up to speed.
Get the drawing board out (your company used app or some free), and try to draw application at high level, so you could see which components talk to which, what they are responsible for, that way you'd get boundaries defined your application or specific parts of code you are interested in operate at.
Now, from the diagram and understanding you have built thus far, you can take a specific component(s) and drill down more and draw yet another diagram(s), but this time at a slightly lower abstraction level.
The important thing to mention, this exercise would be beneficial not just in understanding application/system that is new to you, but also would be useful when you'd need to extend it, maybe integrate with other system.
Needless to say, that would be useful to any new joiners like yourself. Diagrams (think design documents), regardless for how complex or simple features they are, of course best if they are drawn and exercised, meaning discussed with the team upfront, before any development takes place, that way, at least in my experience, is saved a lot of time in development phase, because most of the problems get solved before you even write a single line of code. And you face significantly less surprises. Some you still can't avoid.
