Taken from an excellent person, this is the correct way to put custom ringtones on your iPhone using Catalina.
I have tried and although it can be hit and miss, it definitely works.
Moving a Ringtone to an iOS Device from a Mac Running Catalina
Adding a custom ringtone to an iOS device from a Mac running Catalina, is possible using VoiceOver, albeit a little challenging as it requires drag and drop.
After creating your ringtone, follow these steps to add it to your iPhone from a Mac running Catalina:
1. Connect your iPhone to your Mac with the USB charging cable.
2. On your Mac, open a Finder window to the folder location where the custom ring-tone resides.
3. Open a second Finder window )or tab), where you can select your iPhone from the Sidebar.
4. Return to the Finder window where the custom ringtone resides.
5. Interact with the table or browser, and using VO, navigate to the ringtone you wish to move to the iOS device.
6. Once you are at the tone, press VO-cmd-f5 to route the mouse to the VO location.
Note: You may need to include the fn key in order for the F-keys to perform as normal F-keys.
7. Press VO-cmd-shift-space to invoke the mouse-down action.
8. Navigate to the iPhone window.
9. Navigate to the item that announces the name of your iPhone, how much space is used and its charge level. On mine, it's the item that's one VO-right after the edit field with my iPhone name in it.
10. Press VO-cmd-shift-f5 to route the mouse to the VO location.
11. Press VO-cmd-shift-space to toggle the mouse-up action.
This drops the tone and will immediately copy it to your iPhone. It will show up in the tones section of Sounds & Haptics for your use.