Even if you are genuinely curious, such inquiries from a boss/"authority figure" can easily come off as threatening, so tread very carefully with advice like that.
Indeed - it only takes two or three easy questions like that to become a pattern, at which point every question is code for "guess what you did wrong", which is toxic.
Try instead: I like it. How did you come up with that?
Still passive aggressive. Any person with a brain would see right through that question. If you need to address something, it's better to be direct than to beat around the bush.
I don't think that is passive aggressive at all. In fact, I say stuff like that all the time, and mean it, because I do like what people come up with and want to know how they did it. How else would you express that?