If I'm out for a walk with my iphone and apple watch, are the steps, activity reading, etc. compiled by the iphone or the apple watch? Same thing with an elliptical or some sort of machine workout?
Both... You can look in the health app on your phone to see all the data. Apple tries to combine data from the watch and phone, and has some smarts to ignore data when both are active at the same time.
(There's also some differences depending on what model phone you have. The motion tracking hardware in the phone has improved since the early hardware used around iPhone 5 or so. If you have an older phone it's possible more data comes from the watch than the phone.)
Thanks Cobra! I've been messing with it quite a bit and, although it's hard to figure out where the data comes from, it's extremely helpful (and motivating!)