How to transfer a ringtone from macOS to iOS

With Sync Library (previously iCloud Music Library) enabled, simply move the file into the iCloud Drive > Music folder.

This setting can be enabled in Settings > Music > Library > Sync Library.

Ringtones are m4a files renamed to m4r, with a length under 30 seconds.

The ringtone can then be selected at the Settings > Sounds > Sounds and Vibration Settings > Ringtone list.