You pick a template, edit the copy and fields, and publish, whether that's a standalone landing page or a form meant to sit on a page you already run outside Keap. Templates and examples exist for the common cases, an opt-in page, a lead-magnet download, a simple contact form, so most businesses aren't starting from a blank canvas.
The real strength is the connection back into the Campaign Builder: a form submission can be set as a campaign goal directly, so the moment someone fills it out, they move straight into whatever sequence that page was built to start, no one has to notice the submission and add them manually.