Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
Same trouble here - my child's MacBook Air can't be upgraded to Monterey, so I could not install from office.com or the Appstore.
But thanks to the www, I found a way.
Have a look here: https://macadmins.software/
There I found a list of versions available. Version 16.77 should run on Big Sur and there is a link to the installer for the suite (https://officecdnmac.microsoft.com/pr/C1297A47-86C4-4C1F-97FA-950631F94777/MacAutoupdate/Microsoft_365_and_Office_16.77.23091003_Installer.pkg)
I downloaded the installer and ran on 11.7.9 Big Sur.
Good Luck!