That's called an intranet (which is like the internet but for a building or small area).
The internet isn't really anything more than computers connected to other computers. If you want to make your own internet you can connect two computers whether wired or wireless, and voila. You can host a website locally that only those two computers can access.
If you run BGP, yes. Instead, you can always just build up huge-ass fixed routing tables that are impossible to maintain once you get more than a handful of networks....
I've read a creepy pasta somewhere from this guy's pov, that somehow discovered a computer at one of the businesses they took over as a contracted IT, think car wash or something, and they've ended up discovering a computer in a closet that was networked with other computers in the area to share CSAM and other smut. I forgot how the story ended as it was fairly long, but they were describing the technical non-sophistication of the whole network in great detail. The PCs were basically just networked windows xp workstations or something, without any sort of password protection on them or their shared folders.