Universal Container wants Opportunities to no longer be editable when reaching the Clousd stage. How should a develoiper accomplish this?

Process Automation settings are a broad category, and while flows (part of process automation) *could* be used, a validation rule is the most direct and declarative way to prevent saves based on criteria.

Marking fields as read-only on the page layout is a static setting and does not dynamically change based on the Opportunity's stage.

Flow Builder could potentially be used to achieve this, but it would likely be more complex than a simple validation rule for this specific requirement of preventing edits based on a field value; a validation rule is the most declarative and efficient solution here.

A validation rule can be configured to prevent saving a record if specific criteria are met (e.g., StageName is 'Closed') and the record is being edited, effectively making it read-only for editing.