Short answer
HubSpot's KB: hover over content and click Clone to start from an existing blog post, email or page. Templates, themes and modules clone in the design manager. Using AI needs Breeze Assistant access turned on, and generating landing page content needs Content Hub Professional or Enterprise.
1. In HubSpot
The relevant screen in my demo portal.

2. Worth knowing
Update the URL and SEO fields on clones before publishing.
3. Related
Location landing pages and landing page tests.