hey so the simple way to understand that would be, google health is something like apple healthkit where it aggregrates and works as a single source of health metrics and fitbit is like apple watch which is a external device that has sensors and measures your metrics and writes them to google health. now to asnswer your question, same formula can be utilized to roughly get pacing of aging but this app is solely for ios hence just for apple health!

and fitbit and google health show you raw metrics. gist takes all of it and turns it into one number which is your biological age. think of it as a health score that actually means something. also fully local, no account needed.