A developer can use a validation rule or a trigger to prevent Opportunities from being edited when they reach the Closed/Won stage. A validation rule can check the stage value and display an error message if the user tries to modify the record. A trigger can also check the stage value and throw an exception if the user tries to update the record. Both of these strategies can enforce the business logic at the database level and prevent unwanted changes.
An after-save flow or the Process Automation settings are not suitable for this requirement, because they are executed after the record is saved to the database. They can perform actions based on the record changes, but they cannot prevent the record from being edited in the first place.
References:
* 1: Validation Rules (Salesforce Help)
* 2: Triggers (Apex Developer Guide)
* 3: Run Flows After a Record Is Saved (Lightning Flow Developer Guide)
* 4: Process Automation Settings (Salesforce Help)