microsoft office

Once you subscribe to Office 365, you can download & install older versions including Office v16.66 which was the last version compatible with macOS Catalina. But you must subscribe to Office 365 first.

Here is the download page: Update history for Office for Mac

Big Sur requires a Mid 2014 or later iMac. Microsoft Office is only supported on Big Sur or later. You might want to try the free LibreOffice which can work with Office files.

https://www.libreoffice.org/