Don't think there are apps that do that for free on the AppStore, or even if they appear as free, pretty sure they charge after, at least I didn't find one. Anyway, what I use at the moment is this online converter https://ymp4.download that workes amazingly. From my past tries to find one they were usually full of ads and viruses, so imagine my excitement when I found one that workes properly. I know there are a lot of options out there, but it's the one that workes for me, I use it for downloading youtube videos for my kindergarten kids, I work in a daycare, and it's something that keeps them entertained.