Now which of those is available worldwide?

There are parts of the world where you can get online and on Facebook/whatsapp without even having “internet”. If you pay for Internet, you get access to other websites as well.

Not one of those but XMPP (once upon a time called Jabber) is available worldwide.

So is Matrix.

How does WhatsApp work without Internet?

Some places you get internet, but you’re limited to 3-4 “basic” sites unless you pay for it. Facebook/Whatsapp is often one of those basic sites. Or was.

So there’s Wi-Fi or cellular network, but just filtered?

Essentially, yeah. I only experienced it once a while back while travelling in eastern Europe. There was free filtered wifi everywhere. Some of the locals said they had the same thing, but from phone plugs in their walls.

I guess the closest thing I can compare it to would be TV back in the day? You got the dozen or so free channels if you had an antenna, and then you paid for cable if you wanted the good stuff.