Explanation
This allows you to specify the exact name of the file when running the automation. The File Drop Starting Source is an automation activity that allows you to specify the source of the File Drop as either a Scheduled or Manual event. This allows you to control when the data is imported from the Enhanced FTP file.
To set up their automation to import the data from the file on a nightly basis, Northern Trail Outfitters should use two ways to start their automation:
File Drop Starting Source. This is a trigger that starts an automation when a file is dropped in an Enhanced FTP folder or an external SFTP location.
Scheduled Starting Source. This is a trigger that starts an automation at a specified date and time or at recurring intervals.
%%BASEFILENAME_FROM_TRIGGER%% and %%BASEFILENAME_FROM_FILEDROP%% are not starting sources, but variables that can be used in file transfer or import activities to reference file names.
References: https://help.salesforce.com/s/articleView?id=sf.mc_as_automation_studio_triggers.htm&type=5

Explanation
To send an email confirming opt-in when someone signs up for their emails via a form on their website, a triggered send data extension can be used to capture the form data and deploy the email. The triggered send data extension can be linked to a triggered send definition that contains the email creative and settings. The developer can use the SOAP or REST API to trigger the email from the website form, and the marketing team can change the email creative without involving the developer. References:
https://help.salesforce.com/s/articleView?id=sf.mc_es_triggered_sends.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.mc_es_create_triggered_send_data_extension.htm&type=5