When joining already uwrapped objects, you have 2 options:
UV maps before joining objects were called equally
The default name for UV map after it's created is "UVMap". If it wasn't changed or if both objects have this map renamed so objects still has UV map called the same, after joining it will become one UV map:
All the islands of both objects will be added on this UV map as they were in original maps. So most likely they will overlap and it may be desired to pack and / or organize them in some way.
UV maps before joining objects were called differently
Resulting object will have as many UV maps as there were ones with different names (in example below 2).
All the islands are on their original maps and there isn't any overlapping. This may be desirable in some cases (e.g. baking). Note though that one object may have no more than 8 UV maps.