How is the Internet organized?

Question

Computer networks

Hello!
I would like to clarify one point.
I know that there are major transnational Internet providers (Tier-1 level), which are distinguished by the fact that they own thousands of kilometers of optics on the ocean floor and a colossal customer base. These are companies such as AT&T, Level 3 and Sprint.
There are also national Internet providers such as Rostelecom / Ukrtelecom / Beltelecom,
which also have their own fiber optic backbones, but only within the borders of their country.
I understand that if 3 national Internet providers enter into an agreement and promise a network on the territory of 3 countries, but we all use the global network and can get into the network of both India and Australia.
How do ISPs from all over the world interact to provide their customers with access to the global network? They cannot enter into an agreement on mutual peering with all Internet providers in the world. So how does it work then?

Answer 1

Read https://habrahabr.ru/post/186282/ BGP