Epic has teamed up with several game studios, and made it possible to actually dabble with professional game projects in the form of mod kits. Of course they are a delight for modders, but they are also a valuable learning tool for budding game developers, because you can download the game source and assets in its entirety. You cannot use these assets on your own projects (except to make mods for that specific game), but it is extremely useful to see how the developers have implemented certain game mechanics, level design, level streaming, game saves, multiplayer etc.

All these mod kits are currently on the Store page on Epic Launcher. Just scroll to the bottom. Be forewarned, some of these projects are huge, even 150 GB, but there are smaller ones too. In every case, we recommend opening these mod projects only if you already have mastered the basics of Unreal Engine development, and have some understanding of different UE components and how they interact with each other. Beginner get easily frustrated trying to find out his way among hundreds of files (which are not always very well organized BTW).

You should also know one thing about these mod kits. Each one of them comes with a specific version of Unreal Engine editor, and some of them are made with a version that could be couple of years old.