What to learn first, Unreal Engine 4 or OpenGL?

Question

IT education

On the one hand, I'm interested to know how the graphics engine works in game engines, on the other hand, wouldn't it be useless if you have no experience with game engines at all? What would be better to study first of all with basic knowledge of C ++ (one book on the language itself - Prata, one book on STL - Myers, a course on algorithms - Sedgwick, a basic understanding of Qt (maybe this is important))? I understand that it depends more on what I generally want to do and why I am doing it now, and, in short, I would like to implement game mechanics on ready-made engines, with the prospect of developing into work directly on the engines themselves.