they total it up probaly, per hit impossible

Maybe not max discipline, but enough of it, 15% multishot on all the gear that allows it IS common, definitely people can stack more than 15% elemental (cold/fire/etc) damage as well, and 3 legendary damage buff gems that are leveled high enough is also common. + not forgetting the right damage buff passives, and also elite damage increases. All these are not as rare as you may think.

However still with the above I'm not sure if these contribute to the billions of damage being mentioned. I haven't played my DH in a bit.

Currently I have 5 UE and a Dead Man Legacy. Haven't equipped them yet as I have yet to find the last piece, gloves, or RORG, to have the UE6 bonus.

I noticed that the damage difference between what I am wearing now (hybrid M3, Nat2, UE2: Damage 1mil) - I am getting average 3mil crit damage, whereas from utube video demonstrating player on UE6, with Damage 1.5mil, can deal up to 120mil per multishot.

Is the bonus of the UE6 really that significant due to the multiplier, or am I missing something here?

Ya, this version of M6 is more fun, not like last time, Plant -> 4 sec cool down -> Plant 2nd sentry -> 4 sec cool down.....
almost fall asleep when doing T6 rift...