They use 464XLAT, basically NAT64/DNS64 with some extra cooperation on the OS’s part for backwards compatibility with apps that hard-code IPv4. You get only a v6 address, and your OS basically synthesizes an v4 network on your device in cooperation with their NAT64 router. But all the bytes going from your device through to their towers are ipv6. Talking to a v4-only website uses carrier-grade NAT64 when leaving the t-mobile network.
Additionally, their fixed-wireless product gives you a physical CPE that does the CLAT (NAT46) side of the 464XLAT.
To the local network, it looks like there's native IPv4, but it's translated to IPv6 by the gateway, and sent to the "nearest" NAT64 PoP to be translated back and sent along its merry way.
They use 464XLAT, basically NAT64/DNS64 with some extra cooperation on the OS’s part for backwards compatibility with apps that hard-code IPv4. You get only a v6 address, and your OS basically synthesizes an v4 network on your device in cooperation with their NAT64 router. But all the bytes going from your device through to their towers are ipv6. Talking to a v4-only website uses carrier-grade NAT64 when leaving the t-mobile network.