Honestly, I assumed the stock phone apps are doing this by default. It would be stupid not to? It's like a low-hanging fruit present ever since phones started having multiple cameras on the back.

(EDIT: though the only real confirmation of this being done that I have is, Samsung's "Camera Assistant" addon has an option to enable "2x zoom" as separate button, and the description there states explicitly it's a synthetic zoom made from combining the main and telephoto camera.)

(EDIT2: come to think of it, for almost any regular mode, if I use my fingers to block two of the three cameras, the image is unchanged, which empirically proves at least Samsung is not doing this - which again makes me wonder, why not? It's an obvious thing to do.)

On that note, I don't understand why there's no widespread, genai-free apps for "super resolution" aka. take a video (or a burst photo shot) of a stationary thing and use it to reconstruct a single photo with much higher detail level.

For videos I can imagine the reason would be that video compression is literally doing the inverse of that (i.e. the information you'd need to reconstruct a higher-resolution image is exactly the one that the compressor throws away for 90% of the frames), so the algorithm and compressor would cancel out. But that doesn't explain why this doesn't seem to exist for burst photos.