Managing textures?

Hello, I am starting to learn the engine and blender, but I am in a bit of struggle, because of few questions I cant answer myself.

  1. Where do indie game developers/designers get textures from? Do they download it from the internet/make them by themselves?
  2. What quality it should be, so the game can look good and maintain good graphics? Should the texture be 4k or full hd is enough, or are there other factors?