Is game engine dev real?

I don't think game engines are drawn from a well in the ground, so yeah it's real.

What if I want to make an RPG game with a text dialog that you can scroll down?

Then do it. Or don't, I'm not a cop.

according to my research, 50% of mobile games are made with Unity? Maybe some for the engine, some for pure coding? Maybe that’s their trick?

Maybe. 90% of mobile games are shovelware slop though. Unity makes that easy. There's no trick.