what to place in Levels

Hi, I have been trying to create a game, after learning and doing so for a couple of years… i was kinda stuck with levels. I am confused on the setup of the levels and what should be contained in it. What would be the optimal setup in storing:

  • Main menu
  • main Game screen (travel screen)
  • sub game screens (combat screen in different terrains)
  • overall Map
  • quest book
  • probably others as well

My current approach was to do “everything” within a permanent level"(yes, that was stupid and messy), but that changed when i was doing a sub game screens (for combat). What would be your approach to such game structure as above? Thanks in advance.