I'd like to eventually learn to make a simple 3d engine suitable for a retro FPS. However, to start with I'd like to learn how to make 2d engines for side scrol

I'd like to eventually learn to make a simple 3d engine suitable for a retro FPS. However, to start with I'd like to learn how to make 2d engines for side scrolling or isometric action games. Basically, game dev is a hobby for me so I'm in no rush to actually publish a game. I'm interested in learning nearly every aspect of how games are made. I've already been playing around with Unity and Game Maker Studio a bit. I'm continuing to learn Unity but rather than spending tons of time learning how to use a game engine's libraries I would prefer to learn how these things are built and make one myself.