Microsoft Office Home & Student 2021 on Macbook

Using Sidecar with your MacBook might let you use the iPad as a second screen, for working on Office documents stored on your Mac. But you would be running the Office programs on the Mac, and accessing files on the Mac. It would not give you access to the iPad Word/Excel/PowerPoint apps.

Microsoft has Word, Excel, and PowerPoint apps for iPads and iPhones. It looks like Office Home & Student 2021 doesn't include access to them. You would need to take out a Microsoft 365 subscription.

Microsoft – Compare Microsoft 365 with Office

Microsoft has online versions of Word, Excel, and PowerPoint that run in a Web browser. You would obviously need an Internet connection whenever you wanted to run these.

Microsoft – Use Microsoft 365 apps for free on the web

From the App Store description for the Microsoft Word app:

"To create or edit documents, sign in with a free Microsoft account on devices with a screen size smaller than 10.1 inches. Unlock the full Microsoft Office experience with a qualifying Microsoft 365 subscription for your phone, tablet, PC, and Mac."

The iPad Air (4th generation) has a 10.9 inch screen.

Have you tried just installing ms word on your iPad and see if you can make it work on the basis of your existing Office for Mac?