Often you spend half the day learning about the task, then learn or refresh some bit of required technique or knowledge... then you just do it.
In carpentry, engine restoration, cooking or whatnot... this is typical of early days. Every early carpentry project requires some learning. A new cut, a join or glueing method. As you mature in the discipline, the density of novelty and learning decreases.
"Learning" is a bit of a catch all. Learning about the product, library or API might just be literal information gathering. Other "learning" might be skill building, requiring practice and repetition to "learn." EG, brushing up on regex to deal with some misbehaving code.
In any case, we tend to be pretty resistant to openly practicing, or training ourselves. We prefer to err on the "knowledge gathering" side. Or... we do agree to practice, but adopt a low effort "just practice" mentality that isn't effective. Layer these tendencies onto modern work, where the divide between learning and doing is unintuitive and vague... most of us are not practicing enough to do what we want to do.
"Just build" is, IMO, mostly a hack to make us practice with sufficient effort and persistence for learning to happen. If you have a thing you want to build, you need to practice building it. The proficiency you acquire is often more useful and valuable (even for the task itself) then the concrete output... until a much later stage in the game than we intuit.