IPad – share the Wi-Fi connection of the Mac to the internet with the iPad

The answer you're looking for, though, is Internet Sharing:

1. Go into the System Preferences on your iMac
2. Go to Sharing, then click on Internet Sharing
3. In the dropdown, indicate that you want to "Share your connection from" Ethernet
4. Check the Airport box
5. You have the option to set your network name, channel, and to use WEP encryption by clicking "Airport Options".
6. Finally, check the box next to "Internet Sharing" on the left hand side.
7. At the prompt, click Start.

You should now be able to connect to the internet on your iPad by selecting the network you set up on your iMac.

USB sharing is also now easy and fast: Can I share my iMac's internet to iPhone or iPad over USB?

If you plug any cellular enabled iOS device with Personal Hotspot turned on to a Mac via USB, the Mac will automatically open a network connection using the Personal Hotspot.

So yes, your Macbook Air will be the 1 connection you see.

The second part of your question is a little trickier and the answer is both complex and confusing but I will try to explain as best I can.

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Based on older knowledge with other multiple interface *nix systems is that it will be a complete toss up as to which connection your Mac will use since both active connections could possible provide the same route to the destination for any network query.

I have even seen, on both Windows and Unix networks, traffic coming in on one network connection and going out on another, which rarely works well.

To get round this, in the Network panel you could disable the "iPad USB" in your default location, but then set up a new location where your "iPad USB" is not disabled.

Finally, the order of your devices should mean that if your wifi is above the iPad USB device in your list when both are enabled, it will get priority over the hotspot, but there are still caveats and this could still use cellular data, but of a lesser amount.