Gameplay in UFO: Extraterrestrials is similar to the original X-COM games. The player starts out with a base where they can add buildings, allocate scientists, equip soldiers and vehicles, manufacture equipment, etc. Additional bases can be bought in different countries but are used mainly for housing interceptors. Most buildings cannot be built in bases other than your main base.
This article uses material from the Wikipedia article UFO: Extraterrestrials,
https://en.wikipedia.org/wiki/UFO:_Extraterrestrials which is released under the
Creative Commons Attribution-Share-Alike License 3.0,