If an Over-the-Air (OTA) update fails, your iPhone might get stuck in a boot loop (the dreaded Apple logo screen) or enter Recovery Mode. iTunes or Finder often fail to fix these issues using the standard update method. Downloading the IPSW file and performing a manual "Restore" often bypasses these glitches and brings the phone back to life.
(critical for a clean IPSW install):
Update And Restore iPhone Using iPsw Firmware [MacOS Big Sur] 18 Nov 2020 — ipsw download ios
button (with Option/Shift) attempts to preserve data, but a backup is always recommended. Device Match: If an Over-the-Air (OTA) update fails, your iPhone