If we go back to the origin of the metaphor and look at the foundation of a house and what we are trying to do, we shall never be confused. We build a house on the foundation. Therefore, - We build ON the foundation. - We build UP something on a foundation.

I would say they're a bit different. Build on - adding to a basic idea eg. "We're going to build on the original script to include the new ideas" Build up - create something from nothing, or make something greater eg. "Build up someone's confidence", "Build up the company" Writing this made me realize how confusing the two phrases could be lol

build on your knowledge = " improve your knowledge taking what you previously know as a basis" [[build on top of/build upon]] build up = to collect "She decided to unleash her built-up resentment after ten years of marriage."