You get the full and sub streams as cameras and binary sensors for movement / object detection (one sensor per camera, not separate). The snapshot service works. It can take a few seconds for the main camera stream to load.
I use Scrypted as an NVR on an Intel N100 box which helps with camera video loading times as the streams are “pre-buffered”.
Frigate works with any camera that has RTSP stream
I use amcrest with no issue
Amcrest has HA integration but i dont use it. I think reolink is popular due to price but i prefer amcrest nonWiFi models. I would never buy amcrest wifi model.