I've seen variations of this problem posted many times, but I'm going to consolidate what I tried and ultimately succeeded in solving this issue for the benefit of other poor souls:

ISUUE:
Bosch dishwasher spray arms don't spin, particularly the upper spray arm doesn't move at all. When starting, you can hear water running but once the "filling" stops and the machine starts to wash, upon opening the door, the spray arms have pouring out of them but they are not spinning. There is little to no water accumulated as the bottom of the tub - It should be up the upper part of the cylindrical filter, otherwise there's not enough water volume in the system to make things move.

TEST:
I poured a large pot of hot water into the base of the tub, closed the door and allowed the machine to resume operation. Open the door again and magically the spray arms were happily spinning.

LIKELY SOLUTION:
Water Inlet Valve. Easy to replace. For many on this and other forums it fixes the problem. It didn't for me.

NEXT SOLUTION:
Moved on to the Float Assembly which contains a float and float switch (a two-fer). Very easy to replace but does require pulling the dishwasher out to access it. It didn't fix the problem for me.

NEXT SOLUTION:
Moved on to the Circulation Pump (spendy!). Very involved repair requiring pulling apart the bottom of the dishwasher, but AP Pro video was perfect and if you go slow and take pictures along the way, you can do it. Watch out for the nasty sharp sheet metal when removing the sump...! Sadly, this didn't fix my issue either.

RETURN TO THE LIKELY SOLUTION:
Went back to look again at the Water Inlet Valve and noticed black chunks inside a new'ish filter. A-HA! The water supply hose was deteriorating and causing partial blockage of water flow into the dishwasher. Swapped it out. Fixed!

LESSON:
Check out the interior of the Water Inlet Valve for signs of debris stuck in it.

Hope this helps someone.

Edited by user Monday, October 5, 2020 8:10:28 PM(UTC)  | Reason: Grammar