SETUP
Enable iCloud Drive
iCloud in macOS Catalina, an overview of the key changes - AppleToolBox
Put projects in an iCloud folder
Mac → iOS
Open iOS Files app, iCloud & Scrivener folder
Click on the Project
The project downloads & opens in Scrivener
iOS → Mac
Click on Scrivener (top left) to see all projects
Click Edit top right & click the project
Click Export to make a Zip file