assetlinks.json for your app, on its App integrity page under App signing. Upload that file.
iOS: Apple ships no generator. You need your App ID Prefix (10 characters, under Membership on developer.apple.com — usually, but not always, your Team ID) and your app's Bundle ID from Xcode. We build the file from those and show it before you save it.
See the step-by-step guide in Open links in your app.