Old stock city quarters are almost universally mixed use here, commercial on the ground level and residential above. Living on ground level, with people passing by right in front of your window isn't exactly popular so those shops are increasingly staying empty and it's not just a result of the internet, but also the move of daily goods from small shops to supermarkets (of being size, smaller ones usually still available within walking distance). Unfortunately, some shops get converted to residential (it's higher rent now that the old guard of shops has mostly disappeared), others to small offices, but the most interesting are those extremely specialist ones that can only exist backed by online distribution. And it's not even limited to big places like Berlin. Realistically, a specialist store would be considered out of range if it is not in the surrounding quarters of either work or home, unless need is really pressing, so theres a bit of diminishing returns going on between larger city size and viability of specialist stores.
In Nuremberg, a city much smaller than Berlin I had a model rocket shop (strictly model rockets, absolutely no branching out into RC planes or fireworks) in walking distance, and one specialising into some specific subset off fishing lures, in a region completely lacking any fishing culture (the carp farms that are the traditional fish supply are harvested with partial draining and nets). A sub-kilometer move later, in a quarter developed mostly with residential ground floors, nothing like this exists. Still some empty shop fronts, but maybe your enough to drive commercial rents down to the point where "exotic online specialist with occasional walk-in business" becomes viable.
In Nuremberg, a city much smaller than Berlin I had a model rocket shop (strictly model rockets, absolutely no branching out into RC planes or fireworks) in walking distance, and one specialising into some specific subset off fishing lures, in a region completely lacking any fishing culture (the carp farms that are the traditional fish supply are harvested with partial draining and nets). A sub-kilometer move later, in a quarter developed mostly with residential ground floors, nothing like this exists. Still some empty shop fronts, but maybe your enough to drive commercial rents down to the point where "exotic online specialist with occasional walk-in business" becomes viable.