It probably would not communicate directly with a smaller fitness app other than some major ones like Strava and my fitness pal. You CAN track weightlifting in the Garmin app, and I do, the watch can even auto track your sets and it attempts to auto detect the lift type but isn't always right so I correct them after I'm done. For lifting workout structuring, it probably won't be as good as your lifting specific phone app. Garmin is very good at running and other cardio activities, most smart watches struggle to measure weight lifting.

The watches can connect to a second Garmin app on your phone called Connect IQ which is their app store where they support third party developers to make apps, widget, ultra customizable watchfaces, integration of new connected sensors.

It does not support common phone apps or have incredible advanced phone integration like an apple or Google watch would have but we like that because we already get enough screen time. I'm able to see and reply to text messages though, see and interact with all notifications, control audio/media that is playing and more though so it's not devoid of features at all.