[davidbarker]

I’m not sure if you’re describing the same issue I had, but whenever my MBP (15”, 2018) would restart during an update (and sometimes just a regular restart), I would get the progress bar after entering my password. The fans would then spin up, the machine would get ridiculously hot, and the machine would restart. It was an endless loop.

Usually, I was able to get it to complete whatever it was trying to do and log me in by holding it directly in front of my air conditioning unit (this was as ridiculous as it sounds), but a few weeks ago even that didn’t work. I had to completely wipe the machine and reinstall macOS. It’s been fine so far.

[wpearse]

I just had a similar endless loop on update (Catalina to Big Sur) last week. Apple guided me to do a T2 firmware update which bricked the machine. Sent it in for service.

Logic board was replaced "under warranty" (it's a 3 year old machine with no Apple Care!).