If you want to know what a bare bones engine is, take a look:
Almost all game systems there you’re going to make them yourself in order to develop your game.
Also, get ready to write yourself many many graphic shader programs… It’s almost an Unity 3 reborn lol
Don’t even speak of “gameplay framework” or networking and etc.
(Although small engines are great pick for certain types of projects, Unreal has tons of code that many indies will never use)