How can I connect my iPhone to my imac

You only have to plug the lightning cable to your phone and the USB at your Mac. if your Mac is a USB-C only Mac you have to use a Lightning to USB-C cable or a dongle from USB-A to USB Type-C.

Using a Lightening plug to USB cable

The type of USB plug depends on you iMac.

In order to do what?? The physical connection provided is accurate however you have not told us what you are attempting to achieve.