They have their own antennas. If the internet connection of one router dies, but it has a connection to another Freifunk node, it gets routed over the other one.
Depends currently I use a free one where they are included. Before that I used another one where they were extra and before that I used a prepaid plan where they were extra just data was cheaper.
Would rather use WhatsApp then SMS. Its much more secure and private and doesn't cost money. Although if RCS is an option it could be used instead of SMS or WhatsApp.
Its difficult to export all data.