As I posted earlier in the week, my 2019 MacBook Pro would not start a few days after I updated to Monterey. Nothing that I could do or the Apple Store did got it going again. The Genius Bar person on Sunday claimed the problem was "sand and dirt" inside the computer messing up the internals, but that didn't explain why my relatively new machine went cold dead. That was before postings in the Forum showed this was really due to the Monterey update, so I had to accept Apple's recommendation to have my computer rebuilt (out of warranty, with no Apple Care, unfortunately).
My computer was returned Thursday, and it definitely works. Their form explains what they did including,"We performed a clean installation of the latest Mac operating version verified on your computer." Amusingly, they did NOT do so. They did NOT install Monterey! Instead they installed Catalina. I can only assume that the Apple Repair specialists realized that it is NOT safe to install Monterey on MacBookPro's!