What are the alien base building mechanics in OpenXCom

The flavor text of the "Alien Base" mission mentions that in order to build a base, aliens need to successfully complete several landings in the area. I noticed this behavior in the game: Various UFOs of different sizes simultaneously appear in the same area, and when left alone a new alien base can be found there shortly after. But when some of the UFOs are shot down, no base will be found.

Which ships exactly appear as part of a base building operation and what needs to be done to prevent the base from being built? Do I need to shoot them all down, or are only specific UFOs critical for the base building process? Do I need to shoot them down before they land or can I raid the landed UFOs? When I shoot them down, do I need to clear the crash sites?