It is possible that multiple areas are connected to Area 0 (backbone) through different interfaces on a single router, or they may each use a different Area 0 router. To connect two areas through a single link would be something like switched ethernet, where the Area 0 router and the other area routers are connected on a shared medium. There is also the possibility of using a virtual link so that one area connects to Area 0 through another area.
OSPF doesn't really have anything to do with the public Internet, which uses BGP as its routing protocol.
An area number is simply a 32-bit number, as are IP addresses. It is perfectly valid to represent Area 0 as
0 or in dotted-decimal notation as
0.0.0.0, like an IP address. Just do not confuse the area number, or an OSPF router ID (also a 32-bit number that can be represented in dotted-decimal notation), with an actual IP address.