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After signing in you get a short setup sequence. You cannot skip it, and it is worth knowing why: every field here feeds a calculation. A missing height does not produce a slightly worse number, it produces a wrong one — and the app would rather ask than guess.

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Step 1 — About you

Date of birth, sex, height, and current weight.
Almost every reference range in health science is age-banded. A resting heart rate of 58 means something different at 25 than at 65. It is also required for Metabolic Age and Vascular Age.
Body composition formulas, and several reference ranges, differ. This is the biological input to a published equation, not a statement about identity.
Both feed basal metabolic rate, which feeds your calorie figures. Weight also drives Metabolic Age when combined with body fat.

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Step 2 — Your main data source

Where most of your data will come from. On iPhone this is usually Apple Health, which is the hub your watch, ring or band already syncs into. You are not locked in. This is the default, and you can refine it later — see Choosing your sources.

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Step 3 — Devices

Anything the app can talk to directly, over Bluetooth. Most people have nothing here, and that is completely normal — your watch talks to Apple Health, not to us. Tap through if this is empty.

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Step 4 — Sources

Which of your connected sources to use, confirmed. Again, with one wearable there is nothing to decide.

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Then you are in

The app opens on Today. It will look sparse — see What fills in now.

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Changing any of it later

All of it is editable: Menu → My Profile for your details, Settings for sources, My Devices for Bluetooth devices. Nothing chosen here is permanent.

Next: Health permissions

The step with the trap in it.