When would you use a semicolon vs a colon when dealing with explanations?
For example, in the following sentence, which would be better?

Walker was not only a trailblazer in the medical field; she was also a relentless visionary.
or
Walker was not only a trailblazer in the medical field: she was also a relentless visionary.

I’ve actually been asked about this one before. I haven’t seen the question in the wild myself, so I’m assuming it’s on Khan Academy?

I doubt something exactly like your example would appear on a real, test-day SAT. If it did, though, I’d go with the semicolon because it’s got the conditions a semicolon requires (independent clauses on both ends). Because the second clause completes the “not only___” thought, though, I can see why people would argue that we’re dealing with an explanation that allows a colon.