To find the name for 192.0.2.10, a resolver reverses the octets, appends in-addr.arpa, and queries 10.2.0.192.in-addr.arpa for a PTR record. IPv6 works the same way with each hexadecimal digit as a separate label under ip6.arpa. The reverse zones are delegated along the lines of IP address allocation, from the regional registries to the provider that holds the block.
That is why the hostname you see is chosen by the network operator. A shared web server has one PTR record, usually a generic provider name, regardless of how many websites it hosts. Reverse DNS therefore tells you about the network an address sits in, not which sites run on it.
It is used in mail server checks, where an address without matching forward and reverse records is treated with suspicion, and it makes logs and traceroute output readable. A missing or generic PTR record is normal for web hosting and only a problem for servers that send mail.
Example
192.0.2.10 → 10.2.0.192.in-addr.arpa
2001:db8::10 → 0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa