The firebase cloud function custom token it is used to apply addition claims on the authorization token.
This is needed because in the rules section on each service (firestore, storage, database, etc...) we need those claims in order to validate the incoming request.
Notes: From the rules context we can't call any firebase service. Only in the firestore rules we access the data from itself.
Admins can accept/reject applications and can download any file from the storage section. Admins can post programs.
User can apply to a program. User can upload files in the {userUId}/filename.ext bucket.
After the account is created the user details are submited into the firestore users collection under userUId document key.