Other’s have mentioned fiber optic cable etc, but in reality it doesn’t really matter what physical media is used in any part of the path. Let me explain it a different way, using post office/mail as an example 🙂
Your local LAN is like your local neighborhood. There’s houses on streets. If you want to send mail to one of them, it’s probably easier to just walk on over and give it to them 🙂 Once you want to send stuff further than your local area though, you need something else.
So you get a post office! This is like your router at home. (note many routers these days are combo boxes and include Wifi and modem). In our analogy Wifi would be the post office using drones to send letters to you as you take a stroll around the neighborhood vs delivering to your house, and the modem you can think of as the post office changing “technologies”, going from local delivery vans to long distance 18 wheelers.
So you want to send a letter (“Pls give me Z webp@ge!”) to some guy on the other side of the world. The internet is very much like the post office in the way it works. 🙂
Your local post office isn’t very big, it would not be very efficient to have trucks leaving from your little post office to every other post-office in the world. So they setup centralized mail depot’s (hub sites if you will) in a select few large cities where they will send all mail. (In the “real internet” world, this hub site would be your ISP).
So now your letter is in the hub site. Just like you’d imagine, they look at the address on your message. If the destination is someone within the same country (aka ISP) great! They send it it the destination’s local post office directly. If the destination is in another country, they will look at the best way they can get that letter to that country. Maybe they have relations directly with that country, if so they’ll give it directly to them. If not, they talk to their neighboring countries to see which one knows how to get to the destination country, and when they find one, they give it to them. It may go through 3-5 different countries before ending up in the destination country.
(In Real world, once your message is at your ISP, your ISP will send it directly to recipient if the recipient is also a customer of your ISP. If they’re not a customer, they’ll look in their address book to which other ISP would be the best idea to give your message to. The ISP’s all talk together (automatically) so each one knows how to get to everywhere)
Getting back to our analogy, once your letter is in the destination country, their local post office system will ensure it gets to the destination.
Now, once of the things to take away here, is along the way your letter may have traversed quite a few hub sites, and traversed any number of different travel mediums. (plane/train/truck/etc). In the same way ISP’s have hub sites and can use different means of connecting together both the hub sites and inter ISP (aka inter-country in the analogy) traffic. It can go over microwave (wifi/radio towers), copper wire (dsl etc), and fiber optic cables. Fiber is the most common due to distance/speed/capacity so you find it almost exclusively on the long-haul links (think inter-city planes). 🙂