1) Ships with Asteroid Mining modules will harvest from any asteroid or comet that they are at (Move To), but that asteroid or comet needs to be a colony as well for them to dump their minerals there. They do not automatically put minerals in cargo.
2) You can use Automated Miner on any body to harvest its minerals without a population, but you cannot use Asteroid miners to harvest a planet or moon. Gas Planets or Super-Jovians are a special case where you can't use Automated Miners, but if a ship has a Sorium Harvester they will mine the Sorium and create fuel, no cargo bay required)
3) A Xeno team is only required if there are ruins on a planet. You can find ruins by Geo-surveying planets. You will know there are ruins because they show up in at least 3 places. First, they show up on the planet's info on the System (F9) screen. Second, the map (F3) screen, on the Display 2 tab, shows all known ruins. Third, on the Industry/Research/everything (F2) screen, if you have a colony on a body with a ruin it will show on the Summary tab of that body. If that summary tab shows "Unknown", then you need a Xeno team.