What is the principle behind my voice echoing perpetually when 2 separate devices are used for playback and recording on video calls?
On each individual device, the speaker output can get subtracted from the microphone before it gets sent to other locations. This prevents others from hearing themselves through your microphone. When using two devices in within audible range of each other, the devices cannot subtract the speaker audio from the microphone audio because the information path does not exist. The delay time is the determined by the network delays between the two devices and if the device with the speaker audio is very loud, you can get a gain of more than 1 to the device with the microphone. If the device with the microphone is using automatic gain control and you are not the loudest thing in the room, then you will eventually only hear the loudest thing in the room. In your case, the echo must have been very loud.