i have an iMac, that has internet access via cable (no WiFi).
I know, that my iMac has Wi-Fi access point.
How can I share the internet connection of iMac with iPad?
The answer you're looking for, though, is Internet Sharing:
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?
You can also share connectivity with iOS by USB cable (if you can not use WiFi), but it is little hacky: https://apple.stackexchange.com/a/232937/98580