I have an electric short 40 gallon water heater that is down stairs in our half bath. The water pressure upstairs in the full bath sink, toilet, and shower is much higher than the downstairs. What could possibly be the root cause of that scenario?

Start by removing the aerator at the downstairs sink and see if it is clogged up. If the aerator is clogged up then you'll need to clean it or replace it.

If the aerator is clogged then you can bet that the washing machine inlet is clogged and you could even have a clogged toilet fill valve, but if the toilet isn't a major issue I'd leave that alone.

Otherwise, a difference in pressure between floors could mean that there is a shut off that goes to the 1st floor and it could be partially open or slightly clogged... maybe?

Start with the aerator downstairs and let me know what you discover, OK?

Mark

There are no downstairs kitchen, half bath sink aerator blockages whatsoever. This just started actually today where the pressure upstairs in the full bath room is just noticeably higher still than downstairs.

If you were to take a pressure gauge to your pipes you'd find the same psi up and down. I think you are having more volume up than down due to a restricted mixer in that downstairs bath.