MS OFFICE HOME & STUDENT 2021

Microsoft's site says that it is a "1 time purchase for 1 PC or Mac."

So you couldn't keep using it both on your PC and on your Mac. The question would be whether you could transfer the program to your Mac (giving up the ability to use it on your PC). The answer to that appears to be Yes.

It sounds like you need to use your Microsoft account to de-activate Office on your PC, and use the same account when you install and activate the Mac version on your MacBook. One person seemed to think that you might need to do the new activation by contacting Microsoft by phone.

Microsoft Community – Microsoft Office Home and Student 2021 Transfer to Different Computer

Microsoft Community – Transfer Microsoft Office Home and Student 2021

Here's what Microsoft has to say:

Microsoft Support – Download and install or reinstall Microsoft 365 or Office 2021 on a PC or Mac

Microsoft Support – Activate Office using the Activation wizard

Microsoft Support – This product is already installed on another device

"If you follow the transfer terms in the Software License Terms, you can move Office to a different computer, but you must uninstall Office from the other computer."

Note that what you would transfer from the PC to the Mac – if you choose to do so – is the right to run the program. The Windows code that you have on your PC now will not run on your Mac. You would get a Mac installer from the Microsoft site.

No.

FYI, before doing any transfer, consider the possibility that there may be some compatibility bugs between Office and Sonoma.

Microsoft Community – Microsoft Office Home & Business 2021 for Mac bugs on macOS 14 Sonoma