Mac book Pro 2020 fan over heating/very loud

Long history here teaches that MAc notebook overheating has two significant causes:

Software: Useless anti-virus and so-called "cleaning" apps will cause overheating and are NEVER needed on a Mac. Some apps like the Chrome browser are notorious resource hogs on both Mac and Windows systems and raise temps. Same with client-side VPNs—the ones in TV adverts all the time. All interfere with elegant and automated self-protection and self-maintenance routines you paid Apple to build into the OS.

Usage patterns: Macbook Pros breathe in cool air and exhaust the heated air. On 14-16 inch models, the air intakes are at the intersection of the chassis with the bottom plate. 13-inch models breathe though the keyboard. If any of those areas are blocked, temps will climb, including using a keyboard cover (13-inch) or using the computer on clothing (your lap) or a pillow or blanket.

Hot air is expelled through the hinge area. Make sure nothing on the workstation is within about 4 inches (10cm) of the hinge.

Another thought. Are you using this with an external monitor and the the Macbook Pro in "clamshell" mode? If a 13-inch, the computer cannot breathe well with the display closed. When any size MBP, slipping the closed computer in a "book" position with the hinge down blocks the exhaust venting and cooling air stops moving.

That would not show up if evaluated at a Genius bar.

Here is an article explaining how to keep your Mac within acceptable operating temperatures.

Keep your Mac laptop within acceptable operating temperatures - Apple Support