Resend, Calendly and Google Calendar. Every integration is optional and off until you add a key.
No. Every integration is optional and switched off until you add a key. Everything works without any of this.
Only to the ones you configure, and only for the feature you switched on. Every plugin with integrations has an Integrations screen showing whether each service is Connected, has Missing configuration, or failed — plus when it last succeeded and a button to test it. If a service reads Missing configuration, that feature is off and nothing is running in the background.