According to recent documents filed with the IECEE, the apple watch’s heartbeat measuring technology is going to be based off of the Pulse Oximeter technology commonly used in medical equipment. It also appears from this filing that the watch may be compatible with android devices, however the wording is quite ambiguous.
Snapshot of the IECEE Filing below:
Despite the pulse oximetry functions being FDA approved as early as Fall 2014, it wasn’t until the 2020 Apple Watch Series 6 that Apple finally introduced SPO2 monitoring functions in the apple watch.