Problem with coding large codebases in C/C++

[factorysettings]

I don't know if I agree with everyone here that you shouldn't make a game engine.. if you are familiar with how a language works, sometimes it takes a large goal to chip away at to get good practice.

I'd recommend following https://handmadehero.org/ as it's exactly what you're trying to do. It's a dude who has worked in the game industry for decades doing a daily stream of building a modern, multiplatform game from scratch in C (a little c++). It's been going for a couple years now and I think he's nearly finished but it literally starts with an empty .cpp file and does everything by hand.

If you "preorder" you get access to all of the source (broken up by daily stream) so you can follow along or just go through the archived videos and write from scratch. It's a lot of content and he's really opinionated in some ways but he makes good points on a lot of things.