I've recently ran across a remote shutter for in-built camera app using bluetooth on Android/iOS.
Here's an example link of the product that I found:
Link (Not here to advertise, but to simply understand technology/mechanism behind it)
As far as I know, it is not possible to send remote commands to the in-built camera apps since there is really no way for the in-app camera apps to handle the remote commands.
However, the above video shows that such remote control is achieved using the bluetooth remote shutter.
I noticed no external app needs to be installed to use such a remote shutter device. Is the device simply sending a touch event action to the phone?
I guess my real question is, how would such a remote device send 'take photo' command to the existing in-built camera apps?