This issue arises because Salesforce Field Service calculates availability by combining Operating Hours AND Shifts. If a user has Operating Hours of 9-5, they are fundamentally "Available" during that time. A Shift is typically used to add or extend availability (or define it if using specific settings), but standard Operating Hours often take precedence as the "Base."

Option A is correct. To have a resource's availability defined exclusively by variable Shifts, the best practice is to assign them a "Shell" or "Empty" Operating Hours record (one with zero time slots defined). With no base hours, the system looks only to the Shifts to determine when the resource is working. This ensures that if a Shift ends at 4 PM, the resource is truly unavailable at 5 PM.

Option B is administratively impossible (you cannot automate changing the STM Operating Hours record lookup daily without complex custom code, and it's not a standard practice).