First of all, you have to manage your expectations and be mentally prepared for the fact that you will just not get most of the engine. UE is a massive project (and that is an understatement) and you will be a VERY good student if you just grasp the gameplay framework (core functionality in c++) and a few specific things (if you understand rendering, you may fiddle with the renderer, if you're an audio engineer, mess with meta sounds etc.). If you just look at the amount of code being pushed into the UE repo, you will soon realize that no single person will ever be able to grasp and contextualize even 10% of the engine. It's just not happening. With that out of the way, what I found incredibly helpful was the Ue4/5 cpp game courses on Udemy from Stephen Ulibarri.