[skohan]

It really depends on the games you are making. Not every game needs nav meshes for example.

For things like audio and asset loading, you can get really far building on top of a library like SDL2 or GLFW as a cross-platform system interface, and using some single-header libraries for things like loading models or playing audio samples. There are also great physics libraries out there like Chipmunk or Bullet which neatly encapsulate most of the "hard stuff" in building a game engine.

For something like a 2D platformer for example, it's perfectly reasonable and easier than ever for a solo developer to take on building a small custom engine.