I tried to update an XS running 14.0.1 from a new iMac running 10.15.7. Said the download was corrupted. Tried again. Same result. Knowing the clinical definition of insanity, I tried over WiFi (much smaller download). No problems.
Same download issue for an iPad Pro running iPadOS 14.0.1, are result when updating over WiFi.
Separate issue with automatic backup/sync to the Mac before attempting the downloads: got a generic "can't sync" message. Researched online; almost all the results were about what to do in ancient versions and reference something called iTunes. (I jest, but they did.) Checked Apple's support site: lots of voodoo suggestions, but nothing that sounded plausible. Since the Mac is new, I figured, why not try hitting that "Change Password..." button under Backups in the Finder window (when an iThing is attached). iCloud remembered my old password, I entered and verified a new and better one, and Bob was my proverbial uncle. Is the system ID or something else in hard/firmware involved? Or is it that I now have a shiny, new machine with a T2 chip (caution: monitor all domestic help)?
Anyway, Apple, if you read these fora, that's three (3) things that did not Just Work™.