How is the telephone and internet network connected / how does the infrastructure look like and how does it work? [on hold]

How is the telephone and internet network connected / how does the
infrastructure look like and how does it work? [on hold]

I started researching VOIP and methods to connect phone calls with
computers. But I have came across something I don't clearly understand.
All devices are somehow in one way or another connected together, laptops,
pc's, servers are on the internet and phones on telephone/mobile networks.
But aren't telephone/mobile networks just another tier of the internet?
Shouldn't it be possible to make a call from a device connected to the
internet to a device connected to the phone network for free? Why does it
cost to call to a phone? How can companies measure this? Who get's the
money - the company which is being called, the company which calls (the
customer pays), or both get a split? How does all this infrastructure
work? I am really wondering all this because I cannot find much
information on this. I'm also wondering if there is a "direct route"
(toll-free?) from a internet connected device to a phone network connected
device?