> I live in the US and have never heard of a phone that uses a different technology for emergency calls vs. normal calls. Why is this? Is it worldwide?

In the GSM (2G) standard, and all of its successors, emergency calls are treated as a special case.

Your phone does not simply place a regular call to 911 or other local emergency number.

The pgone instead goes into a special mode where it can connect to any available carrier network to place the emergency call.

It seems to be the case that some older phones didn't support placing an emergency call over VoLTE, so they just fell back to using 2G/3G when that was still available.