how can I use Microsoft Office on MacBook Air?

You will need to purchase Office 2019 or Office 365 subscription from the Apple Mac App Store or from Microsoft.

https://apps.apple.com/us/app-bundle/microsoft-365/id1450038993?mt=12

https://www.microsoft.com/en-us/microsoft-365/buy/compare-all-microsoft-365-products

etc

Patched Office 2016 will also work with recent versions of macOS, but that is no longer sold.

Microsoft Office 2011 and earlier are 32-bit apps and will not work on macOS Catalina 10.15, and later.

If you were using Office 2011 or earlier prior to a macOS update, your Office files are still present.

You’ll need Office 2016 or later, or an alternative.

LibreOffice is a free alternative, and targets Microsoft Office compatibility.

Apple iWork (Pages, Numbers, and Keynote) is also free, and can also read and write many Office files.

If you have, or want to sign up for a Microsoft 365 subscription, you can download the apps, Word, Excel, PowerPoint, and Outlook, from the App Store, or you can do so at Microsoft.com. Or you can purchase Office 2019 from Microsoft.

Office is not free. It never has been.