This is one of the things Apple does right. HomeKit working local is a pretty great setup and just works. I put my HomeKit cameras on a VLAN without internet and device isolation and they still work seamlessly. The hard part is getting cameras that are wireless. I use scrypted but even then, getting ONVIF or RTSP isn’t as straight forward nowadays. I also have a local frigate backup which works great too. You can pipe in the detection to scrypted with MQTT.