Broadcast stations are required to carry EAS messages. You normally hear a test once or twice a month.
Nowdays, it's mainly an internet connection but- I believe the backup is radio stations. Used to be the primary station would broadcast an (EBS in the day) that equipment picks up to trigger the other stations to broadcast.
Take the internet away, you may get it on a cell phone. But the backbone for years has been radio. Take that away…..