On Discord I had recommended NewPipe which is a full youtube-like frontend, that does not require a YT account, and provides extra features like “background playing” of just the audio, and download button for offline view.
@Soup had recommended Vanced which seem to be a more advanced/feature-rich alternative (but I did not try yet).
Quick search for iOS: there is Vanced iOS (did not find NewPipe version).
While I don’t make use of background playing, I have ublock origin on mobile firefox and don’t have to worry about the ads. I do like that built in download button option.