In all of my camera guide books I include a section on using external flash. With the cameras that have a flash shoe, like the Sony DSC-RX100 II, the Sony DSC-RX10, the Fujifilm X10, the Panasonic Lumix DMC-LX7, and others, there is no problem attaching an external flash unit, because each of those cameras has a flash shoe. But some other cameras that I write about, including the Sony DSC-RX100, the Nikon Coolpix P600, and the Canon PowerShot S110, do not have a flash shoe, so there is no way to attach or connect an external flash unit.
With those cameras and others like them, you can use an external flash unit that has a built-in optical slave capability, or you can use an external flash unit along with a separate optical slave device. I thought it would be useful to do a video that demonstrates how this works, so I put together a short demonstration using the Nikon Coolpix P600 camera. Here is the video:
For more information about specific cameras and using external flash units with them, please see my guide books for those camera models.