NYC Mesh volunteer here. We are our own ISP and operate with generously donated transit from multiple providers. Most recently Pilot Fiber donated a 40Gbit uplink to help us kick off our new Brooklyn PoP. https://www.nycmesh.net/blog/supernode-3-is-here/

NYC Mesh pays for a backhaul. I assume that this use is covered in the ToS, but it isn't just a normal residential internet connection.

NYC Mesh pays for it through donations. Last I checked they were trying to setup a 501c3 to manage this, unsure of the status though.

DE-CIX is our donated IXP connection. We connect via peering and transit at an internet exchange point (IXP). We don't pay for an ISP connection.