[typing]

Thing is...
Unlike North America with it's big brand comics that are worked on by like 20 to 30 people doing 7 or 8 comics every month most of the comics in Japan are done in home studios with either a circle of artists friends or sometimes a single lead artist who created the comic idea and 1-3 other artists the publishing company keeps on file to give work to because while good artists, are not creative enough to make their own publishing stuff.

Marvel and DC comics actually produce the stuff the sell, like you said they are comic book *Studios*

Comics in Japan like JUMP and RIBBON are not Studios but just Publishers, so no art is actually done in their branches but artist sell them their work and get contracted for more.

You might be able to go in and see an Animation Studio, while some animation artists do work from home and just deliver their stuff to work at the deadline the physical studio's do exist and people do work in them.


EDIT:
Only way to see a studio in Japan then would be to meet someone who makes comics and then actually go to their house, some artists may rent out another apartment or the Publisher may rent out an apartment for them to work in especially if they're a weekly comic and they have trouble meeting their deadlines.