Authentication and security

Hi,

I have a Xamarin App, I can login with Facebook auth, but I also wish to add email+password authentication.

Here is how it work now
1 - Login is based on Facebook, I receive a "token" back, and store it locally in a "AccountStore"
2 - I connect to my CosmosDB directly, but also on a Azure storage directly, the connection string are hardcoded into the app code.
The same for the other Azure services, like search, storage etc...

So, I have a few questions:

1 - how do I add email + password authentication, I need to register/login/forget password features, but also I need a token back !
I have read that "Mobile App Services" are dead now ! So, how can I do ?

2 - I'm logged into my mobile app, but I connect directly to the CosmosDB, it is insecure, right ?
Is there a way to connect to the azure service wth my token by example ? Or there is another way to do ?
I don't want to put the "server connection string + password" in the mobile app. How can I do ?

Thanks