Delivery Method: WEA utilizes a one-to-many alert delivery service
called SMS-Cell Broadcast (SMS-CB), rather than SMS Point-to-Point
(SMS-PP).
SMS-Cell Broadcast (SMS-CB) is a method for sending short text messages
simultaneously to multiple mobile phone users within a specific geographic
area. It is a feature within the GSM (Global System for Mobile
Communications) standard and is sometimes referred to as Cell Broadcast.
Typically there are laws dictating how emergency alerts have to be delivered, and your phone's OS will follow that. So "it" doesn't override your phone's volume. Your phone's OS will override it's volume when it receives the alert.
Might as well start with the source code[0] and dig deeper:
* CMAS https://en.wikipedia.org/wiki/Wireless_Emergency_Alerts
* ETWS Earthquake and Tsunami Warning System
[0] https://cs.android.com/android/platform/superproject/main/+/...
Is this a universal standard? How does it override my phone’s volume!
Typically there are laws dictating how emergency alerts have to be delivered, and your phone's OS will follow that. So "it" doesn't override your phone's volume. Your phone's OS will override it's volume when it receives the alert.