Apple Health is off until you turn it on. If you never do, everything in the app works exactly as it does now — you just type the numbers yourself.
What is read
Your step count. So a day you spent on your feet counts towards your level without you logging it. Steps are copied into the day's record while the day is still open, because Apple Health knows today and only the app's own log knows last Tuesday.
Your weight. So a reading from a smart scale reaches your chart without being typed in twice. Importing fills empty days only: a number you entered yourself is never overwritten by a scale.
Finished workouts. What the activity was, how long it lasted, and when. So a session you recorded in another app or on a watch is counted here too, and the app stops telling you that you did nothing on a day you trained for an hour.
Hours of sleep. So a night your watch already measured is not a night you have to type again. Time in bed is not counted as sleep, and where two devices recorded the same night the readings are merged rather than added — both of those produce a believable wrong number rather than an error. How the night felt is never imported: Apple Health has no field for it, and it is the more useful of the two columns. Seven hours says little about tomorrow's fatigue; seven bad hours says a great deal.
What is written
Nothing. Not a symptom, not a cycle, not a meal, not a weight. The permission the app asks for is read-only, and there is no code anywhere in it that writes to Apple Health.
Why you are asked four times
Because they are four separate decisions, and iOS shows one sheet per request. You are asked for each where it is actually used — steps at the level ring, weight at the chart, workouts at the workouts card, sleep at the sleep card — rather than all at once at launch, when you have no idea yet what any of them are for. Saying no to one does not affect the others, and you can change any of them later in Settings.
Where it goes
Nowhere. All four stay on the device, in the same local database as everything else you log. They are not uploaded, and they are not part of what is sent when you use chat or scan a meal. The privacy policy is the full account.
Deleting it
Revoking access in the Health app stops future imports. What has already been copied into your log stays in your log — it is part of your record now, and the app does not quietly delete days of your history because a switch moved. Deleting the app removes all of it.