Health app on the phone;
browse/activity/workouts/data sources & access/apps allowed to read data and select Strava.
Here's how to choose the sources that Health uses first:
Open the Health app and tap the Browse tab.
Tap a category, then tap a subcategory.
Scroll down, then tap Data Sources & Access.
Tap Edit.
Touch and hold the Change Order button next to a data source, then drag it up or down in the list.
To turn off a data source so it doesn't contribute any more data for that category, tap the tick next to the source.
Tap Done.
If multiple sources contribute the same data type, the data source at the top will take priority over other sources. Any new apps or devices you add go to the top of the list automatically.