Where do I find landscape mode on iPad air?

Landscape? rotate your device or read this support article:--> Rotate the screen on your iPad - Apple Support

Regards

Giulio

Your iPad’s screen rotation maybe locked.

Toggle OFF screen rotation lock by swiping a finger downward from the upper right corner of the iPad’s screen to reveal the iOS/iPadOS Control Centre.

Look for the button/icon with a white background with a red colored circular arrow with a red padlock symbol inside of the circle.

Tap that icon/button to release the screen rotation lock.

Be advised, many iPhone apps downloaded to an iPad may NOT support screen rotation.

Good Luck!