Why do I have an IPv4 and IPv6 address at the same time?

Why do I have an IPv4 and IPv6 address at the same time?

Because your ISP supports IPv4 and IPv6. They're not mutually exclusive.

… some servers see my exteal network address as an IPv6 while others as an IPv4. Why?

Some servers support both IPv4 and IPv6, so the OS chooses to coect via IPv6 and the server sees your IPv6 address.

Other servers support only IPv4, so there isn't much of a choice; they'll always show an IPv4 address. (Often it's because the hosting company doesn't have IPv6 support, e.g. AWS; but sometimes it's just laziness on behalf of the server admins.)