Another person beat me to it, but it's basically an open source app for wearables and other related devices. It aggregates all of my health metrics and made it so I never once had to connect my watch to Garmin's cloud services. All of my data is just stored locally on my phone.

You can see the supported devices here: https://gadgetbridge.org/gadgets/

an open source app for wearables, its on fdroid. works without contacting manufacturer servers, but for some devices you might still need the official app for first time setup