is there a power on/off on the magic keyboard?

Presumably you refer to the Magic Keyboard for iPad. As the Magic Keyboard for iPad doesn’t have a battery - and is powered directly from the iPad - the keyboard does not have a power switch.

By contrast, the standalone Bluetooth Magic Keyboards designed for Mac (that can also be used with iPad), do have a physical power-switch.

Yes. The USB-C port within the spline of the Magic Keyboard for iPad is for charging the iPad while connected to the Magic Keyboard. This USB-C port serves no other purpose.