Does a reboot of a Mac remove all viruses?

There are no known Windows-like Viruses in the wild that self replicate and affect macOS, because of the underling UNIX  Foundation and Permission Limitation. 

Additionally, and specific to Big Sur macOS 11 and Monterey macOS 12, the Operating System resides in a Sealed and Read Only Volume that can not be opened by the User or “ Bad Actor “ Software.

The Only thing this Antivirus software is protecting is the Bank Account of the Developers and for zero return to the User aside from the problems this software creates.

The The Built in Security  is all that is required to protect the computer.

There is Adware & Malware which will affect the computer and is generally downloaded from “ Shady “ Websites or Developers who Inject this software into the download.

Should “ Certain & Specific Software “ referenced from above be installed - it will negatively impact macOS. It is suggested, to download directly from a respected ASC Contributor, the application Malwarebytes for Mac. 

It is free or paid for added features. 

Run the Application and it should remove the malware / adware.

Once done, restart computer and test.

Hi,

it depends if you download and installed a program that contains a virus, a reboot will not be sufficient.

In this case case you need to install an antivirus and hope it manages to remove the virus.

On the other hand many malicious websites block your screen to get you to pay or download something.

in this case a reboot could help.

Whickwithy wrote:

If I were stupid enough to somehow download a virus of some sort, does a reboot remove it?

A safeboot makes an attempt to remove malware yes.

SafeBoot How to use safe mode on your Mac - Apple Support will sort many anomalies

Takes a bit longer to get to the login screen, does a quick disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, font cache, etc.

Login and test. Reboot as normal and test. Caches get rebuilt automatically.

In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a reboot will put it back to normal mode.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

ref: macOS - Security - Apple

macOS - Security - Apple

ref: Apple Platform Security - Apple

Apple Platform Security - Apple Support

May 2022 update

https://help.apple.com/pdf/security/en_US/apple-platform-security-guide.pdf

Apple uses its Software Update service (which also drives the system software updates that show in the App Store) as a mechanism for installing “background and critical” updates that are installed silently in the background with no notifications to the user.

Whickwithy wrote:

I think you answered part of my question. I don't download anything purposely, period. I just use the software that comes with the Mac. So, I should be safe.

I guess I'm worried about some nefarious software that comes about just by visiting a website. I'm pretty sure, now, that would 1) not happen and 2) even if it did, if I reboot, it would be gone.

A - " nefarious software that comes about just by visiting a website " that is covered the the previous link writeup - the ability of the User to Identify good from " nefarious "

A - Even at that, it would require the Owner of the computer to purposely action this nefarious software .

A - The actions or inactions of the User Choices is beyond our ability to control.

A - " even if it did, if I reboot, it would be gone. " That is inaccurate.

A - Once a " nefarious software " is downloaded , giving permission to install by the User ( you ), it is there regardless of a Computer Restart

One can living in a " Walled Garden " of safety and never download anything.

Or can do some personal homework, read from the previous link Effective defenses against malware and other threats then have a productive and meaningful experience on macOS.

The choice are the Users

Anything on this list look familiar ?

This list is only a Partial List.

Any of the below should be removed as per Developers Instructions 

This will include CleanMyMac , This will include BitDefender , 

This will included Norton Antivirus , Sophos Av Software

Intego AntiVirus, McAfee, MacKeeper, Avast AntiVirus

Ad Guard, Webroot , ESET

The The Built in Security  is all that is required.

Followup regarding downloading only from the Apple Apps Store which is s very good practice.

Yes there are some " Bad Actor Software " which doe appearing on the Store.

The Apple Apps Store is a Business just like any other Business.

In this case, as long as the Developer abides by the Stick Rules of Apple and is prepared to paid Apple a Fee on each Sale - the Application will and can be on the Store.

Apple does not verify exactly what the Application will do on the computer. That is up to the User ( you ) to perform the necessary research on the Application Before purchase.

Example of such an instance is type of " Bad Actor " Application is CleanMyMac which is, on these Forums akin to Malware

Then as per below image - another software purposing to Protect from Virus etc.

Neither of the Two Title are suggest to get even close to the computer ever.

Whickwithy wrote:

I see. So, you consider the tools provided useless.

They work for me.

What "tools" work for you? If you are referring to third party apps that claim to clean, protect, manage, boost performance, etc. they may cause problems.

Some put it Trojans in Garage for you delete and the reboot is because sometimes those Trojans can install bits throughout the system in different folders and even in Main part for all users that require a reboot to get them out! That's why it roots the Mac, just in case!

Exactly what tools.

such a generic statement is less than informative