Survivalist did make a great guide about most of your questions. It has 4 parts:
He also has an app with useful info (based on your questions): https://aoe2-de-tools.herokuapp.com/ This app is useful as answer for question 1.
I like this approach. It is much better then just copying build orders. In the end you cant really play at high level without build order, but intuitively learning the game is for the me the best approach. For that reason these guides of Survivalist really stands out to me. Most other guides are more about learning build orders, but this guide is more about learning the game intuitively.
He has also other great guides, which might help you with intuitively learning the game.