The heart has several muscles and looking at the signal between your hands is a mix of all of them, including EMG signals from your hands, arms, lungs too.
More electrodes are always better for rejecting noise and looking at each muscle's signal. Analog Devices ECG products are very good, Lots of app notes there. That alivetec device probably uses AD8233 etc.
The novelty is they take the ECG signal and make it ultrasonic, which your phone/tablet microphone can then input to their app. No Bluetooth.