So at a basic level, well only talk about routers. Every computer/server on a network has an address. When your computer wants to talk to another it attaches the IP address of the destination computer to every piece of data that leaves your computer saying where that data wants to go.

It goes from your computer to your router which has a table of the addresses it knows (your network at your house) and then an address of another router that it sends everything that it doesn’t know.

It does this a few times before your data gets to a router that says “oh, I know a router that knows someone that knows where that is” and it sends it that way. Until it reaches a router that knows the specific computer to send it to.