Verbs such as 住, 坐, 放, and 站 indicate some sort of position or placement. Because of this, they are exceptions to the general sentence structure and will typically have the 在 after. Unfortunately, there isn't really a clear-cut rule about verb placement relative to 在, so it's just something you have to get a feel for through exposure and practice.

That's just the way the verb "住" is. It's always "住在somewhere."

As far as i understand... General word order (doesn't matter whether it's a question or not) Subject or time - subject or time - verb - object

+1

Not much difference here. The literal translation of 层 is “layer”. 楼 is “building”, and here it can be seen as being short for 楼层 (floor).