Yes: the Watch app runs a whole workout from your wrist, and Apple Health is where Rhythm reads your heart rate, sleep and recovery signals.
On the Watch you can log a set, change the weight or the reps with the crown, start, extend or skip your rest, and add an exercise from a catalogue that lives on the watch itself. It keeps working with your phone in a locker on the other side of the gym, and the sets reconcile when the two reconnect.
With your permission, Apple Health sends heart rate, resting heart rate, heart-rate variability (the beat-to-beat change in your heart rhythm), active energy, sleep, breathing rate, and — on Apple Watch models with the sensor — your wrist temperature during sleep. It also reads protein and water, and menstrual flow and menopausal state, if you or another app record them there, and never writes cycle data back. Rhythm reads workouts that other apps saved to Health too, only so the same session is not saved twice. Live heart rate and calories from a Watch workout flow into the workout summary, and Rhythm writes your finished workouts and their active energy back to Health, with your effort rating if you give one.
Clear and consistent signals mean today's suggestion is offered with more confidence; thin signals, or two devices that disagree, mean Rhythm says so instead of guessing. Your logged sets set the weight. On a day your signals read clearly low, Rhythm can suggest a lighter one, never a heavier one, and the number you lift is always yours to type.
A raw number left on its own. Rhythm turns what Health sends into plain words and says what it means for today, and every reading in the app opens to a “why” in one tap.
Wrist temperature never sets your cycle phase. At most it changes how a cycle estimate is worded, and only with hedged, medium confidence. Your cycle dates and phase are worked out on your phone and are never saved on our servers, whatever you connect. And a wearable is optional: Rhythm reads your training log first.