usage - Difference between Xはどこ and どこがX

I've been absorbing Japanese through osmosis for years, but I just started learning it formally. The app I'm using presented these two sentences with the same translation, I think just to show that the interrogatives like どこ can't take は as their particle:

病院はどこですか Where is the hospital?
どこが病院ですか Where is the hospital?

My question is, when would you use the latter? (My understanding is that the first is more normal.) What's the difference in emphasis?

I think I get the difference for a different interrogator. If I understand correctly, 先生はだれですか is neutral/putting emphasis on teacher, so it could mean "Who (among us) is a teacher?" Whereas だれが先生ですか puts emphasis on "who," as in "Just who is that teacher." (I'm overemphasizing the distinction.) But I don't see how that applies to どこ.

Answer

A) 「病院{びょういん}はどこですか。」

B) 「どこが病院ですか。」

The two sentences above surely have different meanings and usages.

As you stated, A) is the more "normal" or "versatile" sentence. It is used to ask for the location of a/the hospital in a situation where the speaker has little to no idea where a/the hospital might be in a town or area where the speaker is a stranger. In fact, there might not even be a hospital there.

So, the replies to A) can be "Turn left at the next traffic light and go down a couple of blocks...." or "I'm afraid there is no hospital around here."

B) is used very differently in that both the speaker and listener know that there is a hospital (nearby). Suppose you are looking at a campus map of a medical school that you already know has a hospital attached to it. You want to know exactly where on the campus it is located and the other person might point a finger at the south-east corner of the map and reply 「ここが病院です。」 or just 「ここです。」.

A がｰanswer to a がｰquestion.

It could also be a small town or area to ask question B) in a situation where you have a prior knowledge that there is a hospital there. Either you may have had that knowledge for a long time or you have just learned it a moment ago.

Finally, A) can often replace B), but B) could not replace A) nearly as often. That , of course, is because B) is more specific and A), more general. By "specific", I mean to say that more specific situations/contexts are required to naturally utter B) (as my example of a medical school map above).