What an embedded address is
IPv6 has 128 bits of address, so a mechanism that needs to remember an IPv4 address can store its 32 bits inside the IPv6 address at a fixed position. Dual-stack sockets do it so an application sees one address type, NAT64 does it so a translator knows the real destination, and the tunnelling protocols do it so a packet can find its IPv4 endpoint. The prefix identifies which mechanism is in play.
The decoder works from those positions. For the mapped, compatible, NAT64 and ISATAP forms the IPv4 address is the last 32 bits. For 6to4 it is the 32 bits immediately after 2002. For Teredo the server address sits in the second 32 bits and the client address in the final 32 bits, inverted, with the inverted port in the 16 bits before it.
Reading each result
Every match shows the IPv4 address in large type, the mechanism name with its prefix in brackets, and a one-line note: for the mapped form, that dual-stack sockets use it to represent IPv4 peers; for NAT64, that it names the IPv4 server an IPv6-only client is talking to; for 6to4, that it is the public address of the 6to4 gateway rather than the end host.
The classification pill comes from the address's leading bits and is independent of the extraction: Global unicast for 2000::/3, Unique local address for fc00::/7, Link-local for fe80::/10, Multicast for ff00::/8, Documentation for 2001:db8::/32, plus Loopback, Unspecified and IPv4-mapped. A 6to4 address classifies as Global unicast, which is correct, while the result beneath tells you it is a tunnel.
The ::ffff: addresses in your logs
The most common reason to use this page is a log line such as ::ffff:198.51.100.7. A server that listens on an IPv6 socket without the IPV6_V6ONLY option accepts IPv4 clients too and records them as IPv4-mapped addresses. The client is an ordinary IPv4 host. 198.51.100.7 is the address to block, allow-list or investigate, and IP lookup will tell you who owns it.
NAT64 addresses are the mirror image. On an IPv6-only network a connection to 64:ff9b::c633:6407 is really a connection to 198.51.100.7 through the carrier's translator. If your network uses a network-specific prefix rather than the well-known one, the decoder cannot recognise it, because any global prefix could be in use. The last 32 bits are still the IPv4 address, though.
When there is no IPv4 address to find
Most IPv6 addresses on the internet today are native: a global address beginning with 2 or 3 was assigned by a provider and has no relationship to any IPv4 address. The info box in that case is the correct answer, not a failure. To find the IPv4 address of the same host, resolve its name with domain to IP or query its A record. The A and AAAA records are independent.
The same applies to unique local and link-local addresses, which are internal and never carry an IPv4 address. If a result looks wrong, remember that 6to4 and Teredo addresses encode gateway and relay addresses, not the end host, and that Teredo's client address is only meaningful if the address really came from a Teredo client. To convert in the other direction, IPv4 to IPv6 produces all the forms from one IPv4 address.