It unfortunately involves CGNATted IPv4, but it's either DS-Lite (https://datatracker.ietf.org/doc/html/rfc6333 https://datatracker.ietf.org/doc/html/rfc6333) w

It unfortunately involves CGNATted IPv4, but it's either DS-Lite (https://datatracker.ietf.org/doc/html/rfc6333 https://datatracker.ietf.org/doc/html/rfc6333) where the end-user router emulates a dual-stack network but encapsulates IPv4 traffic on IPv6 from the perimeter to the CGNAT device at the ISP or just plain NAT64/DNS64 (https://datatracker.ietf.org/doc/html/rfc6147 https://datatracker.ietf.org/doc/html/rfc6147) where IPv4 traffic is relayed at dedicated IPv6 addresses operated by the ISP with the help of special DNS (which might be the GP meant for IPv6-only networks, but it tends to be unreliable for a lot of reasons). In practice DNS64 is now being removed from a majority of networks because some specialty applications (I say "specialty" but these are work VPNs, conference systems and the like) reacts badly (because usually they can't understand IPv6 in the first place), replaced by either DS-Lite or plain dual-stack (possibly with CGNAT for IPv4).