No audio 
So what exactly does that base unit do? does it act as a server for local or does it act as a router only for the cameras?
or both
The cameras communicate to the base unit, and the base unit communicates to the internet, uploading whatever the cameras catch.
Which batteries do these take? The rechargeable or the CR123?
The specs say “4 CR123” batteries.
How is it powered? Four CR123 batteries
Are these compatible with apple HomeKit?
I have these already, and they do not work well at all. Motion detection is awful.
Does anyone know if you can add more cameras? Max of 5 cameras maybe?
It does have audio, you just can’t talk back thru it like the Pro model can. Placement for best results as far as motion detection goes is within 10ft of the desired area. I had bad detection myself until I moved the cameras closer to the place I wanted focus on.
And yes you can have 5 cameras for free. Anything more is an option but it requires a monthly fee.