How are the two ISPs connected in this diagram?

Question

Computer networks

There is a scheme where two groups of computers (A, B, C, D, E, A', B', C', D', E') are connected by a network. For example, the first group of computers wants to send a photo of a cat to the second group. Computers A, B, C, D, E are connected by a common router (as seen in the picture). The router leads to the modem. And the modem, in turn, with the help of a special cable leads to ISP1 (Internet Service Protocol num. 1).

Question: ISP1 and ISP2 are connected by some kind of "dense black line" . Is it some kind of cable / wire or is it related to data transmission by signal?

Answer 1

This is the Internet, or rather the main communication channel.
Most often it is optics.