One can’t just pick all the source code and study from there… it is an overwhelming task this way, there are people which is working with UE4 since 2014 and still can’t tackle 60% of its entirely, not because they are lazy, but because the engine itself is in constant change and receiving new technology and tools on a very regular basis. I can’t figure why one would want to study this way, unless want to recreate his own game engine, another daunting task for one person.
I recommend this:
a) choose an area of interest : animation, materials, visual fx, film, tools, etc
b) create an idea for a small project targeting your area of interest
c) start to look which techs you will need to gather to realize your project and how those are mapped into the engine
d) start building the project, researching while you feel you got stuck on some point
e) watch Epic tutorials and live streams (past recent ones are very useful and don’t watch things before 2017, too many changes already)
I hope you can get there and success!