Hmmmm...it's whatever Option+k does on macOS. I thought it was the degree sign because it doesn't actually combine, but apparently degree is option+shift+8 instead.
An acquaintance just about had a nervous breakdown recently because they somehow used ˆ (option+i, used for e.g. î) in a regex instead of ^ and couldn't figure out why such a basic thing was failing.
Ha! I had been thinking Option-K like Kelvin (also a unit of temperature--but ironically one that isn't properly a "degree". Maybe that should have been a hint!)
Option-zero has a tiny underbar for me, which I think is in Bulgarian(?) abbreviations.