According to Songfacts, Gomez declared:

‘Good For You’ is actually one of the first songs I recorded for the record. It was stunning – it was a piece that was just everything I feel a woman should embody, but not in the obvious way. I vibed with it. The track only took about 45 minutes because I just did it all the way through. I didn’t want to stop in between. I wanted to feel the whole song as a whole, and they were there, and it was just a beautiful start to the album.

She also said in an interview with Elvis Duran on Z100’s morning show :

I think that this song represents the confidence that I truly have inside of me, and I think it’s the vulnerable side that I’ve expressed, but it’s also the combination of just feeling myself.

She also said to iHeartRadio:

That’s a gem. Honestly, I think it was the second song I recorded for the album and it just … It was not even a beat. It was just a skeleton of a song, and it’s ultimately what I feel women can feel like. In the video, there was a shower scene, there was a white t-shirt scene. What I view women to be like is just beautiful, and being confident in yourself, saying I’m a 14 karat, and you just want to embrace that feeling. I have become more comfortable with my sexuality, and who I am, and my body, and I feel really proud of that. That’s why it was the one to kick off the album.

According to Songfacts, Gomez told Elvis Duran that putting a feature artist collaboration on the track was an organic process:

When I recorded the song, there was no feature. I don’t even think there was a plan for a feature. I recorded the track, I responded to it so much that I had the entire team who wrote the song come to Mexico and finish the album with me because it was the perfect way and the perfect tone that’s set for this next chapter for me. And then once it was done, I could not help but want somebody on it. And I’m obsessed with A$SAP Rocky’s new album. So I slipped it to him and next thing you know he responded really well.
When I got ‘Good For You,’ it was just the melody. We didn’t know what we wanted the sound to be like. Some of the melodies are very soft, so it could have been a little more pop but I didn’t want the production to take away from the vocal.
I’m a huge fan of hip-hop, urban – that’s just my taste. I think that’s my natural direction. When I was recording the whole album, everything I was writing or recording was all mid-tempo. I was drawn to that, I guess. I think now more than ever I’m comfortable and confident going into the studio knowing what my vision is.

Rocky explained on his behalf to MTV News how he changed the song after Gomez' people reached out to him:

Me and my boy Hector [Delgado], we added to the beat. We added instruments and s–t – more bass, snares. I started singing on it. Produced it. Wrote it.

He also claimed:

I don’t think it’s a different approach [of music]. You just go in. Go for the kill. I mean, it wasn’t the type of song you try to go into… It was just like me telling her, you sure you wanna go here with me? I gotta change the beat. I gotta talk my s–t. My jiggy s–t.