How rDNS technically works in Digital Ocean?

Hello, I wanted to ask how rDNS (reverse DNS) exactly works in Digital Ocean, it’s unclear to me how simply naming my droplet to an FQDN would automatically handle the PTR record(s), what if I used an already existing or non-existing FQDN? What if I named my droplet “google.com”? What if I named it “abcxyz.org” (assuming such domain doesn’t exist)?

In other words, how is rDNS technically/internally set up for DO droplets?