Remove or edit a synthetic.conf file in etc/
To edit or remove synthetic.conf when it is causing catalina not to boot or login to your user, you must first boot into Recovery mode (hold down CMD+R during he boot sound). Then from the Utilities menu open the Terminal.
If you have FileVault enabled you will need to unlock the boot drive (there are other questions with answers on how to do this from the terminal so I wont repeat those here).
Provided that you have unlocked the boot drive you should be able to use the ‘find’ utility to locate your synthetic.conf like this:
cd /
find / -name synthetic.conf

For me it was inside a ‘private’ folder.
Then you can delete it with ‘rm’ or edit it with ‘vi’ if you’re feeling brave; I just deleted mine. Turns out that if you put spaces in it, instead of tabs, it causes your system not to boot on Catalina - What a great feature Apple :-)