I want to theoretically understand the general relationship between the PSTN (public switched telephone network), cellular networks, and the Internet.
I understand that the PSTN provides the infrastructure that ISPs buy in order to offer provide internet subscribers with shared lines.
What about cellular networks and how Internet is accessed over a mobile phone? Are cellular networks, with all their stations and satellites, also ultimately connected underground with the PSTN? Is the PSTN therefore the lowest level of communication for all networks?
Or is it that cellular networks are independent of the underground telephone lines and exists fine even without it?