I have a strong programming background just not from game development. I only made some pong and snake in high school and I did some OpenGL in college.

I want to make my own game engine. Nothing fancy just a simple 2D game engine. But because I'm kinda old school and feeling retro. I want graphics to look like old 8 bit games (megaman, contra, super mario, ...).

So how were the old games made back then? I want the simplest approach. Were they also using assets (images) like newer engines now do? How do you achieve this kind of rendering using OpenGL?

Keep in mind. Simplest solution. I want to know how it was made back then and how I can replicate that. Doesn't even have to be OpenGL. I can draw on window canvas. I do want to make it from scratch basically.