2020 27-inch iMac continuously shuts down and restarts

To add to KiltedTim's recommendations I offer this:

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these articles:  Pubic VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu).    

Additionally a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.

Also Tim's absolutely correct in the comment about CleanMyMac. After removing it and bitdefender according to the developers' instructions you can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For aforementioned software you'd do the following search(es): 

1 - Name contains cleanmymac

2 - Name contains macpaw

3 - Name contains bitdefender

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

If you get warnings that the file can't be deleted because it is in use or used by another app boot into Safe Mode according to How to use safe mode on your Mac and delete from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Also Malwarebytes is the only "anti" app I use. It was developed by a long time contributor here and a respected member of the internet security community.

EtreCheck will not show heating issues but will show apps and processes known to raise internal temps through workload. Usually the processor will clock back if internal sensors detect too much heat. However, a thermal shutdown remains the ultimate safety action.

If you are not running any useless anti-virus or one of those client-side VPNs that get hyped in TV adverts, the usual cause of overheating in that class of iMacs is workstation arrangement. You iMac must be able to take in cool air and expel hot air.

Cool air intake:

The entire bottom edge of you case has air intakes:

That roughly three inches (75mm) of clearance between the lower edge of the display and the workstation surface must be respected. We had another iMac thermal shutdown complaint and the person posted a picture worth a thousand words:

As the guy on TV says, "Well, THERE"S the problem!"

That area must be fully uncluttered.

Hot air exhaust:

Heated air is expelled through a small exhaust vent inconveniently located behind the top of the iMac 27 stand:

Note that the vent is about the same width as the stand.

I never thought of this as an issue. Then I worked an iMac 27 heat complaint here where the poster eventually revealed that the iMac was mounted ABOVE eye level with the screen tipped fully forward. I tested that positioning on my iMac and found that, in the fully-tipped position, the stand almost completely blocked the exhaust and raised internal temps rapidly.

Has I not been able to reproduce heating on my own iMac, I'd never belived tha to be an issue.

So check that workstation as well.

At this point, you might be better off to just back up your data, follow the instruction here, then set it up from scratch and restore your data.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If you're using the LaCie drive as a normal external drive formatted either APFS or OS X Extended (journaled) then the LaCie software is unneeded and can adversely affect the systems managing of the drives.

If the above is correct then you should uninstall the LaCie software according to the developer's instructions and confirm complete removal with Find Any File.

Please post a more up to date EtreCheck report and we can look it over to ensure all the malware (Bitdender, VPN, CMM etc...) are now totally gone.

Download and run etrecheck and post the report it generates here.

https://etrecheck.com/en/index.html

It is a diagnostic tool written by one of the senior members here. It will provide invaluable information as to what's going on with your Mac. There is not personally identifiable information in the report.

That will give us a good start on figuring out what's wrong.

Get rid of bitdefender. It is also garbage on a Mac.

There are no known viruses in the wild that can infect macOS.

Malwarebytes isn't harmful, but it serves very little purpose if you have not disabled Apple's build in protections.

Get rid of the VPNs. They serve absolutely no legitimate purpose. They do nothing to protect your privacy or secure your data.

Exactly the opposite.

+1 to Kilted Tim's advice to backup and then do an erase and install of Mac OS. Then restore your data files and manually install all third party apps. Your system likely has been made a mess of due to the third party security (Antivirus, Cleaning, VPN, Security, Maintenance) apps that you installed to the point that the ONLY way the computer will be stable again is to erase and start over again. Yes this is a lot of work however the lesson that should have been learned is NEVER EVER install any of those types of third party apps.

All that Mac OS requires to remain stable, fast and secure is simply keep it up-to-date and restart the computer about 1x per week. Other than that please leave Mac OS alone!

Here’s what I found out after trial and error. I had exactly the same problem and found out that the chips on this thing run hotter than ****! What you need to do is download an extension to control the fan (there are a few available) set the fan speed to around 80% of full and on all the time, then forget that you ever had a problem! My Mac was crashing 7 times a day and now it hardly ever crashes. Good luck!