Dragging music file from macbook to iphone deleted entire music library on iphone

Hello. I have a new Macbook air and an iphone 16.

I'm trying to (re)create a personal ringtone that I had on my old iphone. I've made a 20s wav file in GB on my macbook from a particular song I want to use, plugged in my iphone, and then dragged the file across to my phone using the finder. I feel certain, although it was a while ago now, that this is what I did previously to create a ringtone, converting the wav file to m4r on my phone, and then setting it as my ringtone.

However, doing this has resulted in the wav file appearing in my music app on my phone, but the entire existing music library of several 1000s of songs was deleted from my phone at the same time. I'm now re-syncing to get the music back on my phone.

I would welcome any advice on what I did wrong, and if there is a better way of going about this.

Many thanks