I never use WA, but I'm typically reaching for fairly basic math calculations that don't involve units. Last example was calculating an estimate cost of DynamoDB based on the peak RPS I'd found, which is just basic math with a lot of significant digits.

Siri draws answers from Wolfram Alpha, so maybe Spotlight does, as well.

Spotlight uses a private framework for mathematical expressions called Calculate.framework. (Fun fact: it seems to be based on Bison.)