Answer 1

1 ) study game loops, methods, graphics pipeline, input stack (event handling), containers (arrays, lists, sets, maps, vectors), iterators for those containers, math (physics, geometry, trigonometry preffered),
EXTRA: learn music and drawing.

2) Study your game engine and how it handles things. Learn the game loop (init, input, update, render), its graphics rendering, physics engine (if any), collisions (should be in the physics engine or category).
Ask yourself...
Will you need buffer strategies?
Will you need system API for windows and inputs?
Can this engine handle my game idea?

Beginner C++ game engine, so i hear (never used it), is SFML https://www.sfml-dev.org

I use SDL for C++ games, which is what SFML is built on top of (so its more advanced, not beginner friendly, but not too difficult) and if you want 3D, OpenGL or Vulkan (extremely advanced; you have to type every little thing and learn shader languages x.x).