Basel Bad. Bf is still in Switzerland 
but it is the tariff border between SBB and DB. From there, all DB tickets are valid, including Interrail for Germany.
To get to Badischer Bahnhof, you'll still need another ticket. that doesn't need to be an interrail; it can also be something else, such as a GA or a local TNW ticket.
The German Interrail is also valid in most local trains. Check the link "Validity of your Pass on non-DB trains” at the bottom of this page. It is not valid in city buses, trams or metro.
In DE its rather complicated if you want-like most Schwyzerli-want to know the finest details. It is NOT valid like your GA. Its trains only, not just DB but about any train-except some very few minor private branch lines and also NOT FlixTrain. It includes the S-bahn in most cities, but again with some exceptions. It is never valid on lokal Stadtverkehr as MetroTramBus. On bus only if they run as ´Schienenersatz´ for raillines out of service due to works. Going to other countries-as far as the border point-as LUXembourg is now free, you could go there.
There is also a germanrailPass which supposedly also includes a few out of DE lines where DB runs its ICE trains-like as far as BRUssel/BE. Conflicting report if that is available to Europeans or only out of continent.
Sector Basel SBB-Bad is indeed just out of validity.
For a complete map-on most stations in DE there are Schutz-Natur maps.
It includes the S-bahn in most cities, but again with some exceptions.
In which German S-Bahn is Interrail not valid? I can't think of any, to be honest.
The German One Country Pass doesn’t exist any more, but Interrail sells the German Rail Pass, wich is the same, but you can also use ICE services to Bruxelles/Brusel via Liege, all trains from Basel Badischer Bahnhof, all trains from Salzburg Hbf. and on the DB-ÖBB Eurocity on the Route Munich-Innsbruck-Verona-Venice/Bologna is it also valid, only on the DB-ÖBB Eurocity (in Austria and Italy).
ÖBB Nightjet will be not be included (???)
The German One Country Pass doesn’t exist any more, but Interrail sells the German Rail Pass, wich is the same, but you can also use ICE services to Bruxelles/Brusel via Liege, all trains from Basel Badischer Bahnhof, all trains from Salzburg Hbf. and on the DB-ÖBB Eurocity on the Route Munich-Innsbruck-Verona-Venice/Bologna is it also valid, only on the DB-ÖBB Eurocity (in Austria and Italy).
ÖBB Nightjet will be not be included (???)
Yeah ÖBB Nightjet is a own topic with German Railpass and currently i wait for a answer of Eurail / German Rail & Austrian Rail.
