Hi CentaurMSP-
Great question. I faced the same issue because I have a 2015 MacBook Pro and is equipped with Thunderbolt 2 ports along with standard USB ports. Your iPhone 11 came with USB-C to Lightning cable. You will not be able to use that cable with your iMac because USB-C (Thunderbolt 3) is different from Thunderbolt 2. Yes you can get an adapter from Amazon to use the cable that came with your iPhone 11.
What I do, when I need to create backups or charge my iPhone or transfer data, is to connect my iPhone with the standard USB to Lightning cable that I have laying around the house. If you this cable then you can use the standard USB port on your iMac and connect the lightning port to your iPhone 11. The iPhone will be able to backup, transfer data and charge itself (at a slower than USB-C rate but will charge). Try that.
Axel F.