How does SOS calling work?

I was driving northbound on I-5 in Seattle in my Mazda 3 and slowed for traffic ahead. A car slammed into the back of my car with great force. I was really shaken up and surprised that my phone called 911. I have front and rear cameras to protect myself (at the suggestion of an adjuster after a ride share driver hit my car last summer). When I review the front camera footage, I can see that the hit adjusted the front camera angle downward. I saw that my Apple Car Play display showed "Emergency Assistance Activated" and "An Emergency Assistance Call will be placed in 10 seconds." I was not looking at the display because I was trying to get over to the side of the highway, and was surprised when suddenly someone said, "911, what's your emergency?" I explained what happened and a State Trooper was there within five minutes.

This is a wonderful feature. How does this work? Did my watch trigger the call on my iPhone? Does the watch also report my location to the 911 Operator? Sometimes after a crash, people are too injured to talk on the phone, so location reporting would be great.