Is it:
Cell tower: "Every phone currently connected to this tower -- display this message now <emergency alert xyz123...>"
or
Cell tower: "subscriber 202-555-1212 known to be connected to this tower, display this message now <emergency alert xyz123...>" , "subscriber 202-555-1213 known to be connected to this tower, display this message now <emergency alert xyz123...>", etc...
or
AT&T central: "deliver to AT&T subscriber 202-555-1212 special text message <emergency alert xyz123...>", "deliver to AT&T subscriber 202-555-1213 special text message <emergency alert xyz123...>", etc...
or
FEMA central: "deliver to mobile subscriber 202-555-1212 special text message <emergency alert xyz123...>", "deliver to mobile subscriber 202-555-1213 special text message <emergency alert xyz123...>", etc...
Is it like a loudspeaker or a high priority special messaging channel?
I have no idea but I would guess there were choices about whether to optimize for speed of delivery versus guaranteeing that every single phone would get the message. And does the alert get stored for phones that weren't on? Or you only get it if your phone happened to be on and connected at the time of the alert or within 1 minute of it? How do they tell whether a phone received the command, or should try again?