In any given mesh of peers, or for example the NYC mesh, suppose I am a mesh client with no ISP. How do I get internet without using another mesh client's ISP connection?

Edit: can't reply because rate limited or something. My point is that relying on good will doesn't scale.

I thought mesh is supposed to involve hopping from neighbor to neighbor - not everyone directly connected to an ISP. What is the difference then besides that it is volunteer-run with donated bandwidth?

While many neighbors could be connected directly to a traditional ISP, in a hilly/mountainous area, only a few nodes may be able to connect directly to an ISP, and this will connect indirectly accessible nodes to the Internet via only intermediary nodes. This is not theoretical - the test network in Oregon does this.

There's payment (out and also in), so it doesn't rely (entirely) on good will - running a connectivity-important node can even net you payments!