Take this song, for example. My first impression was that this song involved Ariana singing on a Zedd instrumental. But then I discovered Max Martin helped write the song, and i'm sure he had a heavy hand in producing it as well. We know from Max Martin's other productions/compositions, that synthesizers are very much in his wheelhouse. So what exactly did Zedd need to contribute?
1 Answer
Disclaimer: I don't really know house/techno producer culture, and my answer will therefore lack nuances involving that.
Taking the "ft."/featuring at face value, this means that the featured artist (in this case Zedd) contributed to the performance/interpretation of a smaller portion of the piece. If I trust that Zedd provided the instrumental, this means that Zedd called the final shots on what notes he played/provided and was rather likely to have played them personally himself - regardless of how many of his notes were directed by Max Martin.