How to create a custom iPhone ringtone using MacOS 10.15 Catalina?
'Create ringtone' part for Catalina (and older macos versions as well) is pretty simple and straightforward if you are comfortable with terminal. 

MacOs comes with built-in afconvert. For example, to convert mp3 file 'mysound.mp3' to ringtone (.m4r file) mysoundringtone.m4r:

afconvert -f m4af -d aac mysound.mp3 mysoundringtone.m4r


Need to be in same directory with source file or pass path along with filename(s).

'Upload part': one can connect iPhone to computer and drag-and-drop ringtone file to iTunes while it's on iPhone 'General' tab. After that locate newly created ringtone on iPhone in Settings > Sounds &amp; Haptics > Ringtone (above built-in ringtones, separated by line).