afaik military and likely police radios dont talk to a central server or anything like in the world of internet. hence some things logical on the internet ate very impractical if not impossible or too risky (single points of failure).

its an interesting domain but hard to get solid info on unless you are working on these types of projects or for some MoD somewhere. most info out there on the net is about old tech.

as far as i know preshared keys are common. hard to rekey ofc in case of compromise so likely they have some tricks up their sleeves to make sure if for instance a unit is overtaken by enemy not all coms are compromised by this key in the device. (guesswork here ofc..) dont think much of this stuff uses priv/pub keys and https or vpn like auth schemes etc.