https://learnopengl.com/ is the usual go-to resource for OpenGL things. You’ll probably want to brush up on your matrix and vector maths if it’s been time since you worked with them, too. 2-D games are usually not too heavy on the mathematics; It depends on the game, of course (mostly comes down to how advanced you want your physics to be).
For a 2-D game, you’re going to want some sort of batch renderer system (combining all the quads into a single mesh to avoid too many draw calls). This playlist by TheCherno covers it quite well, though make sure you don’t just blindly follow the videos.
Probably not relevant to a top-down game (again, depends), but these posts (though old) have pretty good explanations of 2-D physics in games.
Also, is there a specific reason why you don’t want to use SDL’s built-in renderer? I’m all for DIY’ing this sort of thing, but OpenGL can be difficult if you’re a beginner. What language are you planning to use?
Feel free to post here should you have any questions; I would happily answer them.
Edit: spelling.