Why do my steps/walks on strava not show up here like they should
No, they should not. Garmin Connect only shows steps logged with a Garmin device. And the syncing of activities works only from GC to Strava, not in the opposite sense.
Only steps made with a Garmin device will show up in the connect app. The connect app can only write into the Apple Health App, not read. (Which I don't like, since I track my water with a different app and always have to manually track it in the GC app)
