I can only answer for the one in my neighborhood:
https://www.openstreetmap.org/way/260790219
This is actually an old tram car converted into a beer garden sales counter,
it serves the area in front of it during summer afternoons and evenings,
but this area is a shared space that also is part of the market place two
times a week.
So only the old street car itself is tagged as beer garden, but not the
area in front of it where it’s tables and benches are placed during their
opening hours.
And to make it even more complicated: there’s a second beer garden
area on the north end of the market place, but this one is permanent
during summer, and is offering lunch on market days, too.
This one is served from within the building next to it, and the facility
inside the building is tagged separately, as even though it mostly
serves the beer garden only there is also a small area for drinking
and eating inside, and sometimes they also serve for events in
other meeting areas in the building.
So “it’s complicated …”
