PTR records live in special zones: in-addr.arpa for IPv4 and ip6.arpa for IPv6. The address is written in reverse so that delegation follows the way address blocks are allocated: the PTR record for 192.0.2.10 is stored at 10.2.0.192.in-addr.arpa.
Those reverse zones are controlled by whoever holds the IP address block, usually the hosting provider or ISP, not by the owner of the domain name. To set a PTR record you normally use the provider's control panel or ask their support.
Mail is where PTR records matter most. Receiving mail servers commonly check that the connecting IP address has a PTR record and that the name in it resolves back to the same address (forward-confirmed reverse DNS). A mail server without a matching PTR record is much more likely to be rejected or treated as spam.
Example
10.2.0.192.in-addr.arpa. 3600 IN PTR mail1.example.com.