For PC/Mac
Zwift workout files are saved under Documents/Zwift/Workouts/YourZwiftID as “.zwo” files.
If you have downloaded a new workout file and want to use it in Zwift, simply save it to Documents/Zwift/Workouts/YourZwiftID (where “YourZwiftID” is your Zwift ID number), then start up Zwift. Your workout will appear under the “Custom Workouts” category in the workouts list.
To further organize your custom workouts you can create directories under Documents/Zwift/Workouts/YourZwiftID and save your workouts to specific directories.
For iOS
See How to copy workout files to Zwift for iOS.