How do I control a CCTV by android phone?

 guess you could be able to do so by following steps (Assuming you are talking about wireless, web connected CCTV camera) -

1. Try to get IP or atleast IP range for the CCTV camera (Try some ip scanner app for listing devices).
2. Try default username or password, or Google for any vulnerability with that model.
3. Write or ask someone to write an exploit for that in form of Android app.
4. Do whatever you want, once you have access to device after exploiting.

In case its some IoT stuff, relying not on Wifi network and instead uses its proprietary communication protocol or frequency, you might need to get info on that to hack into it.

This is what I know insofar, on how it works. I maybe wrong, so please correct me if so.