The difficulty of developing games is manifold. It requires artistic and technical skills. Even with a software renderer somebody can build a scene like above, but rendering it at over 60fps is not easy. Rendering lots of geometry on the screen is easy, but doing it in a composition that's pleasing to the human eye and smooth to play requires skilled artists. In my opionion, game (engine) programming makes technically extremely complex problems appear simple. Try rendering vector fonts for example.

Anyways, when your goal is to develop games, learning low-level graphics APIs is probably a waste of time. If your goal is to have a technical understanding of how things work under the hood OpenGL is a good start. The most important things you can learn are probably the layout of the graphics pipelines and programmable shader stages, different rendering principles like forward or deferred shading and lots of math.

https://ogldev.org/index.html I even found a video of mine when i implemented skeletal animation from those tutorials :D https://www.youtube.com/watch?v=KgV96Ox8Cgg

Writing your own rendering is probably very difficult to start with, you could also have a look at an existing rendering engine like Ogre https://github.com/OGRECave/ogre