I used what some call a Framework, an SDK, an API. It gives me primitive access to the OpenGL/Vulkan graphics handler.  The Framework I use is SDL2. 

As an example, the animation for the player in 'Idle' mode is 4 frames. I wrote all the code that takes the X,Y coordinates, the frame, the time between frames, to show the animation. In an engine like Godot for instance you pick the images in your sprite sheet. You tell the animator the time between frames, how you want the frames handled etc. At run time the  animation hander takes all of that information you set up in the animator and runs the animation.  

I have been 'C' coder for over forty years so doing this kind of work is the best part of these jams. I have found in my forty plus years of programming that although an application may be very robust there are still limitations that you will run across and ultimately impede your creativity. Using 'C', a compiler, and a framework  I find that I do not have those limitations.

That being said, I have tried to use all three major engines, Unity, Unreal, and Godot. I found the Godot had the best workflow and intuitive design for making games. Their GDscript is excellent. If I ever decide to switch to an engine it would be Godot. Unity was okay but it is a real memory pig, and not very optimized, but Valheim was written using it so there is that. Unreal has geared everything toward their 'Blueprint' handler and although you can write code in C++, it is at best cumbersome, at worst annoying.

I seemed to have rambled a bit. Good luck in your programming career and I hope you have great success!