to work in/on/at a place

Hi all,

sorry if it a bit silly, but I just wanted to know what preposition
would you use to tell about the place you go to work every day, not
necessarily the exact place but the kind of place, for example:

I work __ a bookshop.
I work __ a hospital.
I work __ a flowers stall at the market.

Or does it rather depend on the type of place?

Thanks.