Given millions of access ISPs, it is not feasible to connect all of them to each other, as it is not scalable.
In fact, it would take O(n^2) connections.
- Access ISPs connnect to Global ISPs.
- Access ISPs may instead be connected to Regional ISPs, which are connected to Global ISPs.
- Global ISPs interconnect via Internet Exchange Points (IXPs), and can also peer directly.
- Content providers / CDNs may run their own ISP network.