AT&T hotspots on car connected plans are on the AT&T network, but on the side on another domain.
Used to be a part of jasper.com, but the again CISCO bought them out Back a while, they were on the same domain as Cricket Wireless (AT&T) owned.
do an "
www.ip2location.com" while connected to your hotspot and i bet the domain is NOT att.com (although managed by AT&T)
Verizon deprioritization depends on what domain you're parked on. My cell phone is on the domain
verizonenterprise.com, so somehow, some prepaid accounts traffic are routed there. I have never noticed a slowdown there. Of course, coverage is coverage, and depends on a lot of factors at any given time.