Explanation
Delegated authentication is a feature that allows Salesforce to delegate the authentication process to an external service of your choice1. When implementing delegated authentication, you should consider the following aspects2:
The authentication web service can include custom attributes, such as user roles or permissions, in the response to Salesforce. These attributes can be used to update user records or trigger workflows in Salesforce2.
Delegated authentication can be used to authenticate API clients and mobile apps that use the SOAP API or REST API login() methods. However, it does not support OAuth 2.0 flows or other authentication methods2.
Delegated authentication does not require trusted IP ranges at the User Profile level. However, you can use them to restrict access to Salesforce from specific IP addresses or ranges2.
Salesforce servers receive but do not validate a user's credentials. Instead, they pass the credentials to the external authentication service, which validates them and returns a response to Salesforce2.
Just-in-time provisioning can be configured for new users who log in with delegated authentication. This feature allows Salesforce to create or update user accounts based on the information provided by the external authentication service3.
References:
Delegated Authentication
Delegated Authentication Single Sign-On
Just-in-Time Provisioning for Delegated Authentication

Explanation
In an environment where Salesforce users are enabled to access Google Apps from within Salesforce through App Launcher and Connected App setup, Google is the service provider and Salesforce is the identity provider. A service provider is an application that provides a service to users and relies on an identity provider for authentication3. A connected app is a service provider that integrates an application with Salesforce using APIs4. An identity provider is an application that authenticates users and provides information about them to service providers3. The App Launcher is a feature that allows users to access Salesforce, connected, and on-premises apps from one location5. In this scenario, Google Apps are connected apps that provide services to Salesforce users, such as Gmail, Google Drive, and Google Calendar. Salesforce is the identity provider that authenticates users and allows them to access Google Apps with their Salesforce credentials using single sign-on (SSO)6.
References: Identity Provider Overview, Connected Apps Overview, App Launcher, Single Sign-On for Desktop and Mobile Applications using SAML and OAuth

Explanation
Configuring an Authentication Provider for LinkedIn Social Media Accounts allows UC to use LinkedIn as an external identity provider for its customer community. This means that customers can use their LinkedIn credentials to log in to the community without storing their credentials in Salesforce. Creating a Custom Apex Registration Handler allows UC to customize how new and existing users are handled when they log in with an external identity provider. This means that UC can control how user records are created, updated, or matched when customers use their social media credentials to authenticate to the community. These two actions can meet the requirement of UC to use social media credentials for its customer community.