I integrated Firebase App Invite in my app as per the doc.
https://firebase.google.com/docs/invites/android
I would like to know how Firebase sends the SMS without mention of SMS in the manifest. I tested it on my phone, and it did in fact deliver an SMS when I added a contact with only a number. Further the SMS appears in the messaging conversation on the phone, which eliminates the possibility of server-side SMS with SMS binds and such.
Firebase Invites on Android is linked very closely to Play Services, which exists on Android devices in a special APK that has permission to do everything. Even though the Firebase Invites SDK doesn't require permission to send SMS, it is communicating with Play Services, which does have that permission. In other words, the SDK is essentially routing the request through Play Services, so your app doesn't need the SMS permission.