Microsoft's policy is to support only the last three versions of macOS. Those are (Monterey, Ventura, Sonoma) – Big Sur fell off the list when Apple released Sonoma last September. As such, I'm not sure whether you can get Microsoft 365 (desktop version) or Microsoft Office Home & Student for your Mac (which can't upgrade past Big Sur).
There is the free office suite LibreOffice. The current version will run on Big Sur.
LibreOffice
There are the Pages, Numbers, and Keynote programs included with your Mac. I consider Pages to be more of a page layout program than a word processor, but you can use it for some word processing tasks.
You might be able to use the Web-based versions of Word, Excel, and PowerPoint. These run in a Web browser (not as native programs. You would need to be connected to the Internet whenever you wanted to use them.
Microsoft Microsoft – Use Microsoft 365 apps for free on the web