What's that phenomenon where as a child you were told a "fact", internalised it, and lived your life presuming it was true until someone points out how obviously wildly untrue it was that you momentarily question reality?

Because I was today's years old when I realised how ridiculous cutting complete blood flow would be. Thank you (genuinely!)

Maybe a bit, but there would have to be a ton of pressure. Consider the fact we have to use a tourniquet to cut blood flow typically, and those hurt like hell.

Maybe. Could be the compressed nerve reduces vasodilation in your fingers. Could be that your finger only feels cold.

Your nerves are responsible for making your muscles move. Blood only keeps them alive, and it takes quite a while for lack of blood to actually matter. A rule for tourniquets in medicine is 4 hours if you want to keep your limb without permanent damage.

A good indicator if blood flow was actually cut off is if your limb is extremely pale (within just 1 or 2 minutes) or later goes blue. But if your nerves are unblocked, you can still move your limb for quite a while even without blood flow. Usually though, if blood is cut off, nerves also are.

Because the nerves are compressed. You move and feel your body via nerves.

Cool, something else to worry about for the rest of my life.

I have problems with my shoulder joint already. I didn't know that arm under pillow is predisposed to this. That's always how I sleep.