How do I clean my old MacBookAir?

What to do before you sell, give away or trade in your Mac - Apple Support

On the MBA sign out of iCloud. Reboot computer and immediately press Command + R. This is Recovery Mode. In Recovery mode choose Disk Utilities and choose Show all devices. Choose the upper most Device ( Probably named Apple SSD something ). Choose the Erase the disk. Depending upon version of macOS installed, format as APFS ( required for Mojave and Catalina ). Make sure it is with GUID format. Back out of DU and choose to Re-install macOS. This will take some time to download. Allow it to install. When it is finished the Setup Manager will appear. Just shutdown the computer via the Power Button.

One all it done - suggest going to Manage my ID on Apples site and log-in. Check that the MBA is NOT Associated with your ID.