The internet is just a bunch of interconnected computers.

You act as a tiny, trivial ISP when you install a WiFi access point. This is sort of like that, but more complicated.

Yeah, in theory most anyone with the know-how, money, legal ability and technical ability can be an ISP. You purchase a bunch of pretty fancy equipment, then you need to find some provider to connect to (the internet is just a bunch of ISPs linking to each other), then provide service to your clients somehow.

That's of course easier said than done. First you need to find an existing place with internet access that will sell to you. Then you need the hardware. Then you need to reach your customers somehow.

The possible roadblocks are that an ISP might not want to cooperate with you, the legal requirements for something like digging streets and laying fiber may be hard to comply with, and equipment is expensive.