By default, all missions start from space, even base generated ones. If you want it to start from a base you'll need to specify an
operationType in the alienMission itself. You probably should check out this particular thread about the topic here (
https://openxcom.org/forum/index.php/topic,6557.msg104669.html#msg104669). Note that if in the example code provided by Kozinsky you also update the mission for STR_ALIEN_HARVEST to use, let's say:
alienMissions:
- update: STR_ALIEN_HARVEST
operationType: 1
It will work, but now, ALL of these instances of the mission will be earth-based ONLY and in this particular example case, require at least 1 alien base present in the region to function or it will be aborted. If your idea is to have bases generate already defined missions but not alter the vanilla "from space" behaviour, you'll have to duplicate those missions to a different name (STR_ALIEN_HARVEST_FROM_BASE for example) and give them an operationType. Also, depending on the type you specify, you might have to set other properties too, all of that is explained in the thread linked earlier.