The initial connection and asset type identification for Windows computer occurs over TCP port 135. You can find a list of the ports used by Lansweeper
here. The Windows domain scanning requirements can be found
here.
After assets are loading, why do they get scanned?
It's not entirely clear to me what you mean by this, what is meant by scanning in the context of Lansweeper, is to retrieve information from the asset and to store it in the Lansweeper database in that computer's record. The computer's record gets initially created in your Lansweeper database at the start of a scan.