The companion to "where does the data live" — this is how the user moves through the wizard and how validation gates that movement.

Navigation state

const [currentStep, setCurrentStep] = useState(0);
const [errors, setErrors] = useState({});      // keyed by step
const [visited, setVisited] = useState({ 0: true });

Validation gates "Next" — not "Back"

• Going forward — validate the current step's slice before advancing. If it fails, set that step's errors and stay put.
• Going back — always allowed freely; never make a user re-pass validation to look at a previous step.

const next = () => {
  const stepErrors = validateStep(currentStep, formData);
  if (Object.keys(stepErrors).length) {
    setErrors((e) => ({ ...e, [currentStep]: stepErrors }));
    return; // blocked
  }
  setVisited((v) => ({ ...v, [currentStep + 1]: true }));
  setCurrentStep((s) => s + 1);
};
const back = () => setCurrentStep((s) => Math.max(0, s - 1));

Error handling per step

• Keep errors in shared state keyed by step (not local to a step component, which unmounts).
• Show field-level errors inline; optionally a step-level summary.
• A stepper/progress UI that marks each step as untouched / valid / has-errors, so the user can see where problems are.
• Let users jump to a step via the stepper only if intermediate steps are valid (or only to visited steps).

The details that signal seniority

• Sync currentStep to the URL (/wizard/step-2 or ?step=2) — makes the browser back button work intuitively, allows deep-linking, and survives refresh (paired with persisted data).
• **Final submit is gated on all steps valid** — re-run every step's validation before submitting; jump the user to the first invalid step if something's wrong.
• Re-validate on back-edits — if editing step 1 invalidates step 3, mark step 3 dirty.
• Unsaved-changes guard — warn before navigating away from a partially-filled wizard.
• Focus management & accessibility — move focus to the new step's heading on navigation; announce step changes.

The framing

"I track currentStep plus per-step errors and a visited map in the shared state. 'Next' is gated — validate the current step's slice, and only advance if it passes, otherwise surface that step's errors. 'Back' is always free. A stepper UI shows which steps are valid or have errors. The senior details: sync the step to the URL so the browser back button and refresh work, gate the final submit on all steps validating — jumping to the first invalid one — and handle focus and an unsaved-changes guard."