What are the technical differences between a bike for off-road touring and a mountain bike? There are a lot of adventure touring bikes, or bikes designed for bikepacking, off-road touring and the like. But what is the difference between one of these and a standard mountain bike.  E.g. what is the difference between the Salsa Fargo frame and something like the Kona Explosif. Or what is the difference between the Thorn Nomad or Raven and an Orange P7. I understand that the touring frames are more likely to have rack eyelets, but what are the other differences in design?
Aside from touring specific braze-ons (rackss, fenders, spokes, pump, etc), the short answer is that adventure touring bikes are going to be more well suited as an all around machine where as an MTB has a narrower performance mark.

The long answer is frame geometry. Briefly looking at the geometry of some of the bikes you listed shows the Kona having shorter chainstays and a higher bottom bracket. In theory giving the bike better pedal clearance a shorter more responsive wheelbase. The Orange on the other hand has a lower BB and longer chainstays for increased stability and better heal clearance for rear panniers.

Frame geometry, however, is extremely subjective. Builders take a lot of things into account when making decisions. A lower bottom bracket for example decreases frame clearance but increases stability at higher speeds by lowering your center of gravity. One frame builder may value this decision higher than the other.