How keys are handled
Add a key in your connections and it is stored encrypted. It is only ever used server-side, is never sent to the browser, and is never included in exported code.
Built-in providers appear alongside anything custom you add, so a service we have never heard of works the same way as one we have.
Using a connection in your app
Attach a connection to a project, then ask for the behavior you want in plain language. The generated app calls the service from its server side using that key.