Other Resources
I'm not the only person to have done something like this, but I think it's important to highlight people or sites that attempt to also demystify Unreal Engine.
Ben UI / Unreal Garden
It almost goes without saying, but this site was a godsend (and still is!) when I need information that isn't easy to get or when I want to understand something that might not get the same consideration actual engine features do.
Xist / X157
This is an incredible resource that one Xist (as they call themself online) has created in their own exploration of Unreal. It's a lot like this site in that it feels much more like one person trying to leave breadcrumbs for their future self. A lot of the exploration centers around the Lyra Sample Game and goes in depth into the systems that that sample game implements and integrates.
Zomg
Zomg has another notable and wonderful set of resources for their own Unreal Engine notes.