If you're interested on a dive into building game engines, I've enjoyed some of Cherno's videos [1] on developing his game engine Hazel [2][3]. It's cool to see the amount of work that has to go into building an engine. I believe Noel when he says building games without a commercial engine is more fun - there's a lot of fascinating optimizations and tricks at the engine layer.
[1] https://www.youtube.com/playlist?list=PLlrATfBNZ98drHSOb-h2e...
[2] https://hazelengine.com/
[3] https://github.com/TheCherno/Hazel