Varvara
for most drones, the Z values *SHOULD* be the orthometric height of the drone, the height above sea level. [BUT as noted above those values can be VERY wrong]
If RTK/PPK they are almost always ellipsoidal height. I have published a map at http://esriurl.com/Map4GroundElevation that can give you both orthometric and ellipsoidal heights if you are interested. (Must log in with ArcGIS Online account to get the Terrain values)
Almost no drones report height above ground (because they don't have a world DEM to know where the ground is). Internally they usually have data regarding height above launch point (so if the entire project is flat, that WOULD be height above ground) but that Z value is not written to the EXIF tags of the JPG images.
Our learn.arcgis.com site has some good training courses for Drone2Map. Since you're new to this, I'd advise you to review our free training.
Cody B.