This particular rabbit hole dive started with this Reddit post:

I have owned macs for around 15 years and I have never had an issue more than this machine. Last night I finally got time to look into why my machine would not update to 10.15.6.

I tried everything: combo installer, safe mode, external USB, clean install. All failed, even the clean install, and I was only able to get back in because I had a time machine back up.

Called Apple this morning and they looked at my logs and said it was the T2 security ship (I hate this thing).

In a nutshell, the Reddit poster was instructed to bring their Mac in to the Apple Store to have the T2 chip re-flashed. Rather than do that, they turned to Google, found this Apple support article, which allowed them to reset the T2 chip.

Worth knowing.