Explanation
An authentication provider is a configuration that allows users to log in to Salesforce using an external identity provider, such as Facebook, Google, or a custom one. When creating an authentication provider, two options that can be utilized are:
A custom registration handler, which is a class that implements the Auth.RegistrationHandler interface and defines how to create or update users in Salesforce based on the information from the external identity provider.
A custom error URL, which is a URL that users are redirected to when an error occurs during the authentication process. References: Authentication Providers, Create an Authentication Provider

Explanation
Identity Verification Credits are units that are consumed when Salesforce sends verification messages to users via email or SMS. To use passwordless login, customers need to receive a one-time passcode via email or SMS that they can use to log in to the customer service portal. Therefore, Identity Verification Credits are consumed with each SMS (text message) sent and should be estimated based on the number of login verification challenges for SMS verification users. Email verification does not consume Identity Verification Credits. References: Identity Verification Credits, Passwordless Login

Explanation
OAuth 2.0 Asset Token Flow is the flow that allows customers to register their devices with Salesforce and get an access token that can be used to create cases. The other flows are not suitable for this use case.
References: OAuth Authorization Flows Trailblazer Community Documentation