The difference between a sketch and prototype is a matter of where you want to put risk. To make a sketch presents a lower risk, but embodies higher risks because the sketched idea may be untested and unviable. To make a prototype presents a higher risk (due to production costs), but embodies loser risks because the prototyped idea can be tested for viability.
Design work is a process of (insightful) failing. I’ve always enjoyed that. But with tools ever lowering the risk of creating even advanced prototypes (cue: Figma and other tools allowing to turn sketches into prototypes), the price of these failures is coming down …maybe also increasing the quality of the final product?