Add NoFollow to External Links

How can you add nofollow to external links in WordPress? This use case shows how a focused WordPress code snippet can handle the task without installing a heavy plugin. The snippet should add nofollow to external links in post content and produce this result: External links inside post content receive rel="nofollow noopener". You can also log in to LeonLab.ai, describe this WordPress use case in plain English, generate the snippet, review the PHP code, and deploy it to the selected WordPress project.