Tags on the building way specify the function of the building - so if it’s a hotel for instance you’d put
name=Hotel X
stars = 5
tourism=hotel
phone=+11 123 456
addr:street=some street
etc

If inside the hotel there’s a spa, for example, then you’d tag this as a node with the appropriate tags
shop=beauty
name=Elements Spa
phone=+11 234 567
addr:housename=Hotel X

A building that is just a supermarket should have all its tags on the building way. If it’s a mall with individual shops, then the building would be tagged shop=mall and the individual shops placed as nodes inside the building.