> According to domain name records, even if a site supports both IPv4 and IPv6 it defaults to IPv6. I would love to know why.

Newer is better? Good clients use the 'happy eyeballs' (rfc 8305) mechanism, which is roughly, open v6, wait a bit, open v4, use whichever connection comes back first. There's a bias towards v6 because often it's a more direct connection, and may work better, but it was also common to have non working AAAA records, so a delayed fallback is preferable to no fallback as used to be common.