English speakers often compare opinions to positions or locations in an imaginary space. For example, you can call a person's official opinion their "position":
Debates and disagreements have "sides", which you can imagine as people standing on opposite sides of a room to show which idea they agree with:
Why are you taking her side?
A. Whose side are you on?
B. I'm not on either side. I'm staying out of it.
And when you ask someone "Where do you stand?" it means "What's your opinion?" or "What side of this debate are you on?" You identify the question or the topic with "on":
It's hard to tell where she stands on this topic.
There's also a noun for talking about where someone stands on a topic. It's "stance". Your "stance" is similar to your "opinion" or "position":
What's her stance on the topic of immigration?
We use "stance" and "Where do you stand?" in business discussions, political discussions, academic debates, and other formal situations.