Ok I tested difficulty modes with the same save doing the same things. It seems to only change some stats. So on very hard when you gather herbs you get 7% or 14% depending on the roll. On hard it's 9% or 18%, on normal it's 12% or 24%.
Enemies change damage depending on difficulty. So a light melee attack on a scrapper on very hard is 28 damage, on hard it's 32 and on normal its 37. Heavy attack is 68 on v hard, 78 on hard and in the 80s for normal. A scrapper lunge attack on v hard takes two whole bars of life, on hard it's a bar and a half and on normal it's one bar. All attacks scale about the same way. I can't tell this for certain but it seems to me that on normal enemies are less agreesive and don't notice you as much. On hard and v hard I can't really tell the difference on their behavior. They don't seem to stun or lose parts of their bodies any more or less due to difficulty.
There are no changes at all to the amount of enemies, the amount of loot, exp gained, and resource gathering outside the herbs. The requirements to create objects do not change. The difficulty can be switched on the fly and the stats all change accordingly, so I believe it's nothing more than a stat adjustment.
Oh and I tested the robot taking loot thing, once they take itbits gone, they don't carry it. This maybe a scrapper only thing as that's what they are designed for, gathering scraps. A watcher did not take the loot of it's dead partner.