Cloud Firebase Functions that power the various aspects of the app. To deploy,
$ npm run deploy --only functions
For sending emails via your firebase functions, you'd need to set your api_key for the service you're using. In this case, we use Sendgrid.
$ firebase functions:config:set sendgrid.key="<API_KEY>"
For this, add a service account key with name service_account_key.json in the lib folder as described here.