When configuring an automation to import files placed on the SFTP, and starting the automation based on a file matching a specific naming pattern, the substitution string that represents the name of the file is%%TRIGGER_FILENAME%%. This substitution string is used to reference the filename of the triggering file in the File Import Activity.
References:
* Salesforce Marketing Cloud Documentation on Automation Studio
* File Import Activity

To personalize the welcome email with the recipient's first name from the Customers data extension, you use the Lookup function to retrieve the FirstName field based on the CustomerKey field.
* Lookup Function: The Lookup function retrieves a value from a specified data extension.
* Syntax:
%%=Lookup('Customers', 'FirstName', 'CustomerKey', CustomerKey)=%%
* 'Customers' is the name of the data extension from which to retrieve data.
* 'FirstName' is the field to retrieve.
* 'CustomerKey' is the field to match on.
* CustomerKey is the value to match in the 'CustomerKey' field of the 'Customers' data extension.
1: Salesforce AMPscript Lookup Function

Content Builder uses the REST API to manage content within Salesforce Marketing Cloud. The REST API allows developers to create, update, retrieve, and delete content assets, making it suitable for cross-channel campaign functions.
1: Salesforce Content Builder API

The optimal SELECT statement for returning all columns in the data extension, including the newly added third field, is:
SELECT*
This statement ensures that all fields, including any new fields added to the data extension, are returned without needing to list them explicitly.
References:
* Salesforce Marketing Cloud SQL Reference
* Query Activity Best Practices