I'm not sure if this is a question about Garmin or ANT+, but assuming a device is Bluetooth enabled, is there documentation for the Bluetooth API? I really want

I'm not sure if this is a question about Garmin or ANT+, but assuming a device is Bluetooth enabled, is there documentation for the Bluetooth API? I really want to find a nice smart fitness thing, but no one makes apps for my phone, so I figure I'd need to make one myself.