How does Canon 7D remotely control the flash?

It is through the camera itself or the wireless signal transmitter installed on the camera to control a flash that is not directly installed on the camera to shoot.

There are many ways to flash wirelessly. One is to use an external wireless signal transmitter. The transmitter is installed on the hot shoe of the camera or the line interface of the flash, and the receiver is installed under the hot shoe seat of the flash, and then the same communication channel is set, so that the flash can be controlled in this wireless way when the camera presses the shutter, which is widely used in studio photography. The other way is to use the active/passive control mode set by advanced flash, such as Canon's flash 580EX. You can turn on the active mode by connecting the camera, and then set several other flash to the passive mode, that is, under the condition of ensuring the same channel and not shielding the signals of the signal receiving end of other flash and the transmitting end of live flash, use the main flash to control the wireless flash of passive flash. Another way is that the camera itself has a built-in signal transmitter, such as most of Nikon's SLR, so that Nikon's or compatible flash can be controlled for wireless flash without using an external signal transmitter and a main flash.