NAT. They use NAT. Network address translation.
Not BIG ISPs mainly MOBILE ISPs.
You get allocated a private IP addres - probably in the 192.168.. range and they translate that to a port on a public ip address.
NAT works becuase there are 64,000 ports available on any originating IP address and since typiucally a computer uses only about half a dozen, you can get around 10,000 computers/mobile phones hiding behind a single IP address. Its very easy to e.g. retyurtn a packket address to PUBLICIP:PUBLICPORT to PRIVATEIP:PRIVATEPORT simply by rewritiung te header. And in the other direction rewriting the sender headers, and storing the mapping in meomorty.
Most desktop users behind a domestic router will be one 'real' IP adress per household with the NAT happening at the router to allow half a dozen plus devices on the home network to use the internet.
It works well and acts as a casual firewall. Only users inside the network can set up a mapping . So outside hackers cannot probe inside your network. YOU have to initiate the conversation.
The trick is to trick you into doing it.