Websites have separate DNS records for ipv6 (AAAA record) and ipv4 (A record). Since ipv6 is not universally supported, the de facto expectation is that if website has ipv6 DNS record, it should also have an ipv4 one for the users who can't talk ipv6.

Your device's network stack first learns if your mobile ISP supports ipv6 or not, and based on that and DNS reply decides how to connect to a given website.