How do I display broken mac book air on a different computer

Assuming it’s only the display that is broken, you can connect an external keyboard and mouse/trackpad, an external display, and connect the Mac to AC power. If the logic board or some other necessary component of the Mac has failed, it will not boot up and your only option is to recover your data from a time machine or other back up, which hopefully you have (and if you don’t, perhaps after this, you will).

You can connect your MacBook Air to another computer using target drive mode via a Thunderbolt or USB-C cable, allowing the other computer to access the MacBook's storage.