Have been thinking about this for a couple years. I have old phones kicking around. Battery shot, hardware dated, but the camera(s) and mic and antennas still work. Would be cool if there were a way to set them up (powered) to stream audio/video or even take stills at intervals (or motion-activated) and then sync the content to the rest of the devices on my network.
I don’t know how complex the programming for something like this would be. But I suspect it’s trivial for those who do know.
It’s not exactly what you’re wanting, but there is Remote Video Camera on F-Droid.
If I were doing it, I would look for an app that just implements the same basic functionality as a network-connected camera. Then video storage, alerting, motion detection, etc. would all be handled by something like zoneminder.