It works like this if you're simply using the Apple Workouts app on your watch, instead of the Strava app. If you use the Strava app, it'll upload directly, as well as, write to your Health data. If you use Apple Workouts, it'll detect the workout for Import (if you don't automatically import), and read that from Health.
Apple Watch -> Health App <- Strava
Strava can directly import workouts from the health app if you don’t like using their app to record your workouts