Itunes

Ringtone sync

Song should be below 30 or 28 secs.
You can crop / trim the song to fit those timelines and export it to AAC format which is .m4a

Make sure you select Manually manage music, movies, and tv shows
Now all you need to do is drag the .M4R file you just created and converted into the sync window. It will sync almost immediately since it is so small. If you have trouble doing this, you can also sync from within the Music app: Select the desired iPhone listed in the "Devices" section of the sidebar, drag the .M4R file we just created, and release it anywhere in the sync window.

Source : https://www.howtogeek.com/469085/how-to-add-custom-ringtones-to-an-iphone-from-macos-catalina/