So, you want to make a game engine (2023)

Unity's recent controversy sparked a heated debate on game engines. Some said that everyone should immediately switch to a new engine, while others replied that switching engines can take months if not years. Some said that the best way is to roll your own game engine, to what others noted that this isn't a simple task. And, you know, everyone is right here: there's no single answer, everything depends on a specific case. But maybe writing your own engine isn't as hard as it sounds? Why would one do that anyway? What does it even mean to make a game engine ?