Do I need separate provider apps and DNS per environment?

You don't need a separate provider app or extra DNS records per Terra environment.

You don't need a separate provider app or extra DNS records per Terra environment.

The same provider credentials work across testing, staging, and production, and users are connected in whichever environment initiated the auth.

If you do want separate provider apps per environment, keep the same custom domain and append the provider app's client ID to the webhook path (e.g. /hooks/{provider}/push/{client_id}). Users are then split per environment with distinct user IDs.