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How is it possible to have CPU with 100% of usage and Memory at 0% of usage at this screenshot (Task Manager showing 1792 logical processors)?

It was a 2Tb of RAM machine I think? So 0.49% of that would be 9.8Gb? Plenty of space and it would still be "0%" used.

How come out of 1TB 9GB is 0%?

Rounding. 0.49% rounds down to 0%.

I don't see why not: create a process that just spawns a thousand threads that infinitely loop, but uses very little memory.

That still is some memory. Threads don't run without consuming memory and some are prone to memory leaks. But, that's not the point here.

It's very little.