Where isHeadphone plug on new iPad Air

Its gone.

You purchase an Apple USB-C to 3.5mm audio adapter and use the USB-C connection for headphones now.

https://www.apple.com/shop/product/MU7E2/usb-c-to-35-mm-headphone-jack-adapter?fnode=1d7ffdcc2f2c2ffe5a4ccf4fdf3ec44d90c654ce649445f48e6ee00604dd9bdb5efc3ec7fe3d4e0d803a95c6fddf2384668568ffb438196988daf77564a484c7a3acaef2cd7b38cee72a1d4276e4a04005483df8cf9ff21613865e61f6b4ca1a

OR

You use wireless, Apple Bluetooth AirPods or other third party wireless Bluetooth headphones, ear buds.

Best of Luck to You!

There is no 3.5mm headphone jack. You can either use headphones that have a lightning connect that plug into the lightning (AKA charging) port or you can use Bluetooth headphones or you can purchase an adapter that plugs into the lightning port that has a 3.5mm jack on the other end.