How do I connect my iPad Air to a ethernet cat5.

You can't.

You buy a Wi-Fi router, connect that to your network, then connect to the router & your network using Wi-Fi on the iPad. iPads/iPhones/etc can only connect to a network wirelessly.

Then follow the YouTube video, I've never heard of such a thing.

PRDavet wrote:

I watched a youtube video thats shows how to do this by using a usb to ethernet adapter from Apple, lighting to usb adapter & a powered usb hub.

Wifi is much easier.

The question is how to connect by wire and not wireless. That's always faster. Use a thunderbolt gigabyte connector