There are not enough public IPv4 addresses for every phone and laptop, so private addresses are used inside homes and offices. NAT on the router swaps private addresses for a shared public one on the way out and maps the replies back. It is why your whole home can browse on one public IP, and also why a device at home is not reachable from outside without port forwarding. NAT is not a firewall, even though it blocks unsolicited inbound traffic as a side effect.