A review agent watches the reviews of your own apps in the App Store and Google Play, drafts replies, and posts them after you approve. It needs developer access to your store accounts, set up once per store.
App Store Connect
In App Store Connect, go to Users and Access, then Integrations, then App Store Connect API, and generate a key. Admin is recommended; a key only for reviews can use the Customer Support role.
Download the .p8 private key. Apple lets you download it only once, so keep it somewhere safe.
Copy the Key ID (shown next to the key) and the Issuer ID (at the top of the page).
On the agent's App Store card, upload the .p8 file and enter both IDs.
Add each app with a name and its numeric App Store ID (the digits after "id" in its App Store link). You can add up to 20 apps.
Google Play Console
In Google Cloud Console, create a service account, create a JSON key for it, and enable the Google Play Android Developer API for the project.
Upload the JSON file on the agent's Google Play card, and note the service account's email.
In Play Console, open Users and permissions, invite that email, choose the apps, and grant View app information and Reply to reviews. Add Edit store listing if the agent should also update your store text.
Add each app with a name and its package name (such as com.example.app). You can add up to 20 apps.
How replies work
The review scanner runs every 2 hours by default; you can change that (see scheduled tasks).
It drafts a reply for new reviews and, by default, posts it only after your approval. You can tell the agent to reply automatically, for example to 5-star reviews.
Google Play replies can be at most 350 characters, and Google doesn't allow replies through the API to reviews older than a year.
Errors
401 from Apple: a wrong Key ID or Issuer ID, or a damaged .p8 file. Check both IDs and upload the key again.
Apple reviews don't appear: the key's role doesn't include review access. Create a key with a suitable role.
403 "The caller does not have permission" from Google: the service account wasn't invited in Play Console, or is missing a permission for that app.
Docs: App Store Connect and Google Play Console.