I would like to use an iPad for playing backtracks (Youtube) in the QC (with and headphone).
Wich insert and cable do I have to use ?
Depending on what iPad version you have (e.g. lightning or usb-c) you would just connect directly from iPad to QC or via hub etc.
This is lightning. So lightning to USB-B ? Is it right ?
Yes, in your case I would just use their Apple camera adapter (or USB hub) or get a cable with lightning on one end and USB-B (as used on most printers) on the opposite end.