domain name system - How are DNS PTR records managed?

As I understand it, an IP address-owner can set up a DNS PTR record for his IP address to map the IP addresses to its host names. There are some things about this I don’t understand. I’ve managed other DNS records often using web based interfaces, but never seen PTR records exposed in those.

A DNS record of type CNAME or A is stored in the DNS zone for a specific domain such as example.com. Are the PTR records pointing at example.com typically stored in the same zone or are they stored in separate zones? If the PTR record is stored in the same zone as as example.com, how does other servers know to query this zone for the PTR record?

What stops me from adding a PTR record saying that resolves to, say, gmail.com? If I manage the PTR records for my IP address, shouldn't I be able to add anything?