How gameplay creeps into every system
The first time someone makes a game, there is very likely no distinction between the game and the engine. This is usually because there is no engine. A common first game would include Ticktacktoe or Hangman. Games like these are simple enough that they can be completely written in main, or possibly using a few functions. They are also simple enough that they don't require complex systems like graphics or physics. There is no need for reusable engine code.
As you are learning to program more, you may decide to try making a 2D game using a graphics API, like DirectX or OpenGL. Code like this can be very difficult the first time it is used, so writing cleanly separated code isn't a top priority. Just as before, the game is made with just a few functions or classes. Drawing code is often mixed with collision code in one single file.
At some point, we all get to a place where the code is too complex and fragile. Hardcoding too many game object types or too...
