Comment by flohofwoe

Many of those "edge cases" lurk in the platform abstraction layer (driver or OS bugs which needs to be worked around), and many of those problems are also taken care of in cross-platform wrappers like SDL (and for 2D games this is completely sufficient, you don't need UE5, Unity or Godot to render a couple thousand sprites and play music and audio effects).

But even more complex custom/inhouse engines are usually not written from scratch, those are often mostly glued together from specialized middleware libraries which solve the tricky problems (e.g. physics engines like Jolt, vegetation rendering like SpeedTree, audio engines and authoring tools like FMOD or WWise, LOD solutions like Simplygon, etc etc...)