Every Canon EOS camera body I've ever owned runs the self cleaning cycle by default each time the camera is powered up and each time it is powered down unless either of the two events occur within a specific time interval from the last time the automatic cleaning routine was ran. So if you turn the camera on and off in rapid succession within a few seconds, it will only run the self cleaning routine when you first turn it on. If you leave the camera on for several minutes, it will run it again when you turn it off. Ditto for turning the camera off after it's been on for a while, then turning it right back on. It will run the routine when you turn it off, but won't run it again when you turn it right back on a few seconds later.

I do not have any experience with the R6 model, but I'd be very surprised if it doesn't follow the same behavior by default.

This can be modified in the menu for at least most, if not all, models. But I've never heard of a camera wearing out prematurely due to excessive use of the self cleaning function, so it makes sense to me to let the camera do it.