Physically, cables in the ground and undersea cables mostly which go through big telecommunications hubs. Sattelite bandwidth is too expensive to be used for most of it.
The devices moving around and directing all the traffic are called routers. There’s different languages the routers use called protocols to talk to each other and there are about 5 major ones to pick from and they have their own advantages and disadvantages. Within those protocols there are different ways to do home and destination addresses so the information can find it’s way to where it needs to go. IP addresses have become the most popular, that and internal codes are how the routers identify and split all the data up into little packets and verify it made it there before reassembling it on the other end.
For the usual traffic, each router builds it’s own table (kind of like a spreadsheet) which lists all the routers it is closely connected to for directing traffic to help speed things up. There’s usually backup routes listed in case the main one it wants to use goes down or the line between them is cut, etc…
The routers all get their own set of rules programmed in to what to allow and disallow and each connection on the router can have it’s own custom speed, protocol, security, encription and rules applied to it. To ease the burden on routers so they can spend their CPU time routing more, firewalls are set up to take care of the security and blocking so hackers have a harder time getting onto routers and being able to access the computers that are more directly connected to it.