it's very glitchy ... I do backups to my iMac via iTunes, running Mojave as a background.
Took several tries and then I got the update, chose to do a download only, that failed several times but eventually worked.
Then tried to actually install and it told me iPhone is up to date.
Then it said SW update is required, for iTunes, installed that.
Then tried to update again and it didn't do anything.
Then it said your iPhone is up tp date.
Went through this several times.
Now it is installing ...
I prefer updates NOT over air, so maybe I need to start backing up to my MBP that runs Big Sur public beta ...