2015 iMac with no Lighting inputs

This is the cable to which Peggy refers, and needs no adapter. That is the kind that came with my Magic KB when I bought a new iMac in late 2018:

Lightning to USB Cable (1 m)

It is also available in longer and shorter lengths:

Lightning to USB Cable (2 m)

Lightning to USB Cable (0.5 m)

The Apple adapter to convert the keyboard costa the same US$19 as the 1m and 0.5m cables.

The Magic keyboard has a lightning port to plug into the keyboard, the other end has a USB connection. I believe current models have a USB-C connection to plug into newer Macs but you can use a regular Lightning to USB-A cable to plug in to an older Mac or a USB charger.