BMW xdrive at least for recent 3 and 5 series is tacked onto a RWD drivetrain, and as such is RWD biased. The driveshaft going to the rear wheels is solid in the sense that it cannot be disengaged from the tranny, it runs straight through the transfer case. Inside the TC they run gears off of it and clutches to power a very simple system + driveshaft to the front wheels, 0-100% engagement. It is NOT a center diff. Rear axle cannot be disengaged so under most conditions max TQ to front axle is 50% when it is locked up. Xdrive electronics sits on top of this and uses brakes + TC to allocate TQ to each wheel. This cannot be turned off, even with DSC/DTC full off. It is like the e-diff feature on RWD, on two open diffs F&R. So it will brake spinning wheels in an effort to get traction either one side to the other, or F/R, diagonally, or all of the above. Supposedly 60/40 R/F split under most conditions, very slow and very fast road speeds excepted when it reverts to RWD.
BMW has since added TQ vectoring rear diffs and some other cool stuff onto newer model AWD systems for some models, not very familiar with them. I am pretty certain there is no BMW AWD system with a true center diff, even X3/X5 have same TC system with solid rear driveshaft thru it.
I am not very familiar with newer Audi systems but they used to all be FWD biased, rear can be (and often is) turned off. They just add AWD pieces to a FWD chassis. No fixed link to either axle (either end can receive no or 100% TQ). Obviously no AWD e-diff type feature on this particular model if it gets stuck with one wheel on the ground. More sophisticated true center diff, could be locked up with press of a button in some versions (I wish my xdrive had that). Quattro has undergone many iterations and improvements. Not sure which one is in use in this demo. These kinds of BMW vs Audi AWD fail vids have been around a while so newer Audis may have fixed this bug.