I've not lived in Germany but visited routinely for longer periods when working for a company there, and the understanding I derived is that Germans have an expectation that the only things (with some exceptions like airport shops) that should be open on a Sunday are things you can do with your family and/or friends - museums, art galleries, restaurants, and, natürlich, stuff like city parks and things that facilitated Wandern machen (going for hikes), so while the grocery store in that small Alps town might be closed on a Sunday, the Seilbahn/cable car isn't.

It caught me out a few times when I'd go to buy a bulk of German confectionary from a Lidl or Aldi to take back home for colleagues and my kids, and of course, I was flying out on a Sunday, so they were closed, so I had to pay airport prices for them.

All that said though, even the stuff that was open on a Sunday would generally shut earlier than usual, because after all, it _is_ Sunday bru, and the trains ran on a sparser schedule which meant my attempt to go to the Eagle's Nest museum, ended up with me getting there just as it closed, and then realising that the next train back to Munich from Berchtesgarden was in 4 hours (IIRC), and my dinner with my colleagues was in 3 hours... But the locals were so lovely and patient with my poor German, and they directed me to catch the next bus to Salzburg, and then catch the ICE to Munich from there.

It blew my Kiwi mind that they're like "Oh yeah, the fastest way back to Munich is to just catch a bus into Austria" real casual like. Schengen is pretty awesome.