Explanation
To enable a new payment type for the Checkout flow, three possible configuration metadata descriptors are needed:
* *.pay: This descriptor defines the payment type name, label, description, icon, and handler class. For example, CO.pmts.CreditCard.pay defines the payment type for credit card payments.
* *.Edit: This descriptor defines the Visualforce page that is used to edit or update an existing payment of this type. For example, CO.pmts.CreditCard.Edit defines the page that allows editing a credit card payment.
* *.New: This descriptor defines the Visualforce page that is used to create a new payment of this type.
For example, CO.pmts.CreditCard.New defines the page that allows creating a new credit card payment.
Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Payment Configuration Settings

Explanation
To have a Contact Us page in the storefront that is a web-to-lead form and has the same look and feel as all the pages in the application, the requirement can be fulfilled by creating a Subscriber Page (CC Page). This is a custom Visualforce page that can be added to the storefront and use the standard header and footer components. The page can also include a web-to-lead form that submits data to Salesforce as leads. Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Subscriber Pages

Explanation
Three actions that a developer must take, in a B2B Commerce store, to accept credit card payments using a client's chosen payment provider are: create a named credential for authentication with the payment provider, create an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface, and create a PaymentProviderGateway record for the custom payment provider class. Creating a named credential for authentication with the payment provider allows the developer to securely store and manage authentication information, such as username, password, token, or certificate, for connecting to the payment provider's API or service. Creating an Apex class that implements the sfdc_checkout.PaymentGatewayAdapter interface allows the developer to define custom logic for processing credit card payments using the payment provider's API or service. The interface provides methods for validating credit card information, authorizing payments, capturing payments, voiding payments, and refunding payments. Creating a PaymentProviderGateway record for the custom payment provider class allows the developer to register the custom payment provider class as a payment gateway in the store and associate it with a payment method. Creating a RegisteredExternalService record for the custom payment provider class is not a required action, as this is only used for registering external services that are not related to payment processing, such as tax or shipping services. Creating an Apex class that implements the commercepayments.PaymentGatewayAdapter interface is not a required action either, as this is only used for D2C Commerce stores, not B2B Commerce stores. Salesforce References: B2B Commerce Developer Guide: Payment Integration, B2B Commerce Developer Guide: Payment Gateway Adapter Interface, B2B Commerce Developer Guide: Payment Provider Gateway Object

A Lightning Message Channel can receive signaling from any component on a page (anywhere on it). A Lightning Message Channel is a service that allows components to communicate with each other across different Salesforce UI technologies, such as Aura, LWC, Visualforce, and Lightning web apps. A component can subscribe to a message channel and receive events that are published by another component on the same or different page. The message channel acts as a mediator that delivers the events to the subscribers. A Lightning Message Channel does not receive signaling only from descendants (i.e., children), only from direct siblings, or only from ancestors (i.e., parents), as these are limitations of event-based communication. Salesforce Reference: [Lightning Web Components Developer Guide: Communicate Across Salesforce UI Technologies], [Lightning Web Components Developer Guide: Communicate with Events]

To authorize the org for Command Line Interface (CLI) access so the developer can deploy Lightning web components, the developer should run the command: 'sfdx force:auth:web:login -r
"https://login.salesforce.com" and then supply the credentials in the browser when it opens. The sfdx force:auth:web:login command is a Salesforce CLI command that authorizes an org using the web server flow.
The web server flow is an OAuth 2.0 authentication flow that opens a browser window and prompts the user to log in to Salesforce and allow access to the CLI. The -r flag specifies the login URL of the org, which is
https://login.salesforce.com for production or developer orgs. Running this command will open a browser window and ask the developer to enter their username and password for the org. After successfully logging in, the developer will be able to use the CLI to perform various tasks with the org, such as deploying or retrieving metadata, running tests, or executing commands. Running the command: 'sfdx force:login -r
"https://login.salesforce.com" is not a valid way to authorize the org for CLI access, as there is no such command as sfdx force:login. Running the command 'sfdx force:auth:web:login -a
"https://login.salesforce.com"' is not a valid way either, as the -a flag specifies an alias for the org, not a login URL. Running the command 'sfdx force:auth:web:login -r "https://login.salesforce.com"
-username'[email protected]"' is not a valid way either, as there is no such flag as -username. Salesforce References: Salesforce CLI Command Reference: force:auth:web:login, Salesforce Developer Tools for Visual Studio Code