I keep mentioning this place obliquely so I figured I should just do a post. It's the Interspar grocery store at Schottentor (near the votive church if that helps). I found this place while noodling around with Google Maps before my stay and then turns out that Rick also mentions it. It's a beautiful old bank building that was converted several years ago and even if you decide not to shop, it's worth popping in if you're in the area to take a look. Unfortunately the outside is covered with scaffolding at the moment but you can still gaze upon the interior.
But you really should shop there, it's more like a Harrods food court operation than a mere supermarket. You can get all kinds of prefab food to go (sandwiches, soups, salads) in addition to the usual food staples you should keep in your room (extra coffee and tea, cookies, candy, chips, fruit, cookies, etc.). Checkout is quick and easy, you either bring a bag from home or you buy one there. If you find something you don't recognize or understand, aim your Google Lens at it and a translation will magically appear. Huge wine departement.
They also have a very nice sitdown restaurant attached which has a good menu at pretty reasonable prices.