"Studio" is American for "zero-bedroom apartment". It's an advertising-friendly euphemism, just like "garden" is American for "below ground level apartment".

An American studio apartment is essentially a hotel room with kitchenette. It has only two doors, one for the bathroom and one to the exterior. Thanks to fire codes, it will usually have at least one escapable window.

While it isn't the lowest tier of city housing, it is the smallest type of apartment with significant supply in the US. Dormitories aren't tremendously common away from university campuses, and communal barracks are uncommon outside of the older military bases--where they are generally mothballed until a particular need arises for large quantities of temporary housing.