iMac 2020 27" retina is randomly powering down

Reset the SMC and reseat the power cord on the back of the iMac during that process.

Reset the SMC of your Mac - Apple Support

Check the RAM modules to make sure that they are firmly seated in the slots.

Install memory in an iMac - Apple Support

If those three things do not help, then run the Apple Diagnostics Test.

Use Apple Diagnostics to test your Mac - Apple Support

If the Apple Diagnostics Test will not run or produces a hardware problem.

Replace the iMac with a new iMac or a Mac mini and external display.

Mac - Which Mac is best for me? - Apple

Does the computer give any advisories when the shutdowns happen? Sudden shutdowns without an advisory can be thermal shutdowns. It internal monitoring systems detect heat approaching the dangers point, they first clock back the processor to reduce the workload. If that does not help, it shuts itself down to save its brain.

Check your workstation area for anything the obstructs air intake and exhaust. The air intake vents are on the bottom edge of the case. There must not be anything of significant height between the workstation surface and the vents. Please do not do what this poster showed in a an older thread complaining about a seriously overheating iMac:

When I worked in law enforcement, we called that a "clue."

The important exhaust vent is on the back between the stand's attachment point and the RAM door. Be sure the iMac is not tipped fully forward. That partially occludes the exhaust vent I tested that tipped position on my iMac 27 5K and confirmed internal heat spikes when the stand is too close to the exhaust vent.

I fully concur with den.thed's recommended actions. Apple Diagnsotics excels at determining the condition and function of the cooling fan and the heat sensors that control the fan's speed.

I might add a Safe Mode boot if those steps do not change anything. I found that Safe Mode is most effective if I let the computer "soak" in Safe Mode for 20-30 minutes before restarting normally. It seems to need time to finish its housekeeping chores.

Start up your Mac in safe mode - Apple Support