Has to do with の being an informal question particle.
Was thinking about asking simple questions, and in the case of saying “the ___ one”, I believe I learned that you’d say something like 「車の中でどれが買いたいですか？」「青いの. 」
But if you ask about buildings, and for clarification the other person responds with something like, “The one with a lot of people standing in front it?”, would ending it with の be grammatically confusing or incorrect? 「あの建物は何ですか？」「人がたくさん前に立っているの？」
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I think your second sentence is seemingly more ambiguous and might be like saying “with people standing outside?”
In reality, it would be “the one with people standing in front of it.”
Explanatory/contextual の would rely on some sort of context to which you’d ask for or supply a reason:
You get up and open the window. I feel like the temperature is just right, but I suspect you might feel differently. 「暑いの？」 「うん。暑いんだ」 or whatever the reason might be: 「ううん。屁をひったんだ。」
If it’s in context, it won’t be confusing.
In a related note, you can also have two の in a row if they’re performing different grammatical functions, similar to how you can have two “had” in a row in English.