Internet is just a series of connected computers. When you pay for internet access you're allowed to connect to your ISP's hub. Your ISP doesn't connect every single computer in the world though, so it has to connect with other ISP hubs. If the hubs your ISP connects to are bigger than their own, they also have to pay for access to those hubs. The biggest hubs are all partnered with each other and have agreed to not charge each other for access.
If you want to be your own ISP then you have to create a hub people can access and then make a deal to access other hubs (which will 100% cost money unless your hub encompasses billions of people). If you've ever had a friend come over and ask for your wifi password, you've already been your own ISP.