1. If you request constructor, the starbase will find an idle constructor and tell it to move to the starbase. If there is no idle constructor, it will then ask a nearby starbase to build one and send it over. This is a handy button that takes a few steps out of upgrading starbases.
2. You must build the appropriate building for the bonus type if you wish to get any benefit. Tourism building for a tourism bonus, etc.
3. As you research more techs you'll get some Military buildings like Military Academy or Hyperion Shrinker.
4. Currently working as intended. The developers might tweak the rules to make diminishing returns on stacking sensors.
5. Yes, in expansions and DLC.
6. Yep. Documents/My Games/Galciv3/Designs is where I find my ship designs.
7. (can't answer, I don't do multiplayer)
8. I am not sure what you mean.
9. Starbases affect tiles that are up to 5 tiles away from them. Certain mid-to-late game techs can expand that area, allowing starbases to mine further away. It is not in any way dependent on your influence, as you can use starbases in other people's area of influence (though they won't like that!)
10. Tourism is automatic - you don't have to do anything to activate it, it just adds income to every planet. Trade, you build a Freighter, then send that trader to a foreign planet. Once it arrives, a trade route will be established and generate income for you.
11. The planet screen shows your total population and your total food. Food is your population cap - if your population equals your food, your planet will not grow in population. If it is under the cap, your population will grow.
12. I think you understand prototypes, you just didn't have the resource you think you did.? I've done that before. Each prototype component requires one resource, and some buildings require those too. So if you put 3 Durantium Drivers on a ship, you need 3 Durantium for that.