What about the 9-euro ticket and the Büsingen exclave on the Upper Rhine?
Can you travel from there to the rest of Germany with the 9-euro ticket?
And if it is possible, how is it controlled that people do not illegally get off in Dörflingen (Switzerland)?
Büsingen is served by the city traffic Schaffhausen (often) and by buses of the SBG (rare). The second is a German service, and the first is a Swiss tariff. But you can take this bus from the German Büsingen to the Swiss Schaffhausen, you have to pay for it, but from Schaffhausen the ticket is valid for the trains of the Hochrheinbahn, even for destinations within Switzerland, as long as it is part of the route from Basel Bad Bf to Singen.
As far as I am informed Büsingen is served by the bus of the city of Schaffhausen and the 9 Euro ticket is not valid.
https://www.suedkurier.de/schweiz/view-bahnfahrer- auf-these-strecken-in-der-schweiz-gilt-das-neun-euro-ticket-doch-nicht-mehr;art1371848,11166965