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.
Except, if everyone idles "hot" because everyone uses Windows 10 then everyone will think their hot idling is actually cold idling.
The only way to really test it is compare Windows idling to Linux idling and if your PC components are slightly more utilized under Windows than Linux, it is possible that difference is due to MS doing something for their benefit with your hardware.