Everyone knows OS and software, browsers, etc all use way more RAM than they used to, and arguably more than they rightfully should. Bloatware, etc.
I saw something recently that alleged MS puts a lot of shit into PCs that say they are doing one thing but are really doing another.
It would be wild if they are basically using the PCs of the world to do something like crunch numbers, mine bitcoin, etc. Oh, hey, this guy has 12 gigs of ram and is only "using" 4, that's 8 for us to sap.
If you have even an ounce of knowledge of how your PC works, it would be extremely easy to spot this.
For the record, I trust Microsoft exactly 0%. Windows 10 will likely be the last Microsoft OS I'm ever forced to use, and only because Visual Studio is still the best IDE out there.
That said, even process monitor will let you know if your computer is doing something other than idling. Your fans will let you know if your computer is doing something other than idling. If your computer is doing heavy number crunching or bitcoin mining, you will know it because it will sound like a leaf blower and performance will be shit.
So yes, they're absolutely spying on you; probably every damn keystroke. But no, they almost certainly aren't using your PC as a Beowulf cluster node.
The XZ Utils backdoor was a lot more subtle than this, and it was still exposed in about a month. I do have my suspicions that the Recall feature is spyware - I don't trust Microsoft to process screenshots of everything you do into a compact digest and not to exfiltrate that digest - but that's a process that makes no secret of what it's doing that is 90% of the way to being up to no good.
Was patched about 2 years ago