I used my iPhone to set up my 12.9 3 gen iPad and many items transferred

I used my iPhone to set up my 12.9 3 gen iPad and many items transferred, contacts, notes, some setting ,etc. However, no iTunes library! How do I transfer music on my computer to the iPad? How do I backup my iPad and sync it to my iMac as I do my iPhone? It has a usb-3 connector while my MacBook Pro 2015 only has usb and thunderbolt ports.