How can I fetch emails from shared email address (or inbox) through an app or through Graph API?

You need to add/consent the required permissions for the user (you can try it out on Graph Explorer by selecting the permissions from "Settings Gear" on left side.) Select Mail.Read.Shared from the list of permissions and consent it. On successful, you can access the shared mailbox and its messages like the following: https://graph.microsoft.com/v1.0/users/{sharedmailboxmailaddress}/messages

Assuming a user token, then you are using delegated permissions. Accessing as an app is a bit different in perms but similar approach. -E-