New to all this and have some questions

This is my first time trying to make a game. I have no knowledge, but have for the last 7 or so days watched tons of youtube videos and messed with simple stuff. Made some materials and texture maps. messed briefly with a simple landscape model. I had a few questions that I can’t seem to find answers for on youtube if anyone can help.

  1. The game I have in mind is a sci fi game and I want my planets to have their own map. Can I create these separate maps in the world comp mode or will they all get mashed together?
  2. Buildings. Do I create the prototype in sperate level and once done and finalized move it over to the game map?
  3. I have mission, but like in elder scrolls they won’t become unlocked until certain requirements are met. How do I code this in so that the game doesn’t present the mission until its time to?