Everything goes into Apple Health but individual apps can barely talk to one another. Oura won’t pull heart rate data from Apple Health, so when I track my lifting workouts on my watch (a ring is a no-go for barbell work) it’s not there. Same for my Polar chest strap which I use during judo and BJJ. Oura is completely blind to this despite the data being available and it having access through Health.
Strava only pulls in workouts for apps that have been directly connected to it. So I have to have n:m connections between apps with this model, and only get sync between apps that have explicit Strava integration. For Garmin cycling workouts this works okay. When I integrate with Polar workouts, Strava insists on reduplicating the data back into Apple Health a second time. I have yet to find a way to get the data into Strava but have it recorded once instead sold zero times or twice.
Garmin directly refuses to use anything not recorded from their own devices. It won’t import sleep from my Oura or Withings trackers, heart rate from anything else, etc. Want to use any of the derived metrics in their app? Sorry, you'll need to exclusively use Garmin devices for everything in order to have all that data in place.
The only thing that isn’t a complete shitshow in this entire ecosystem is Apple Health.
