You have the pieces. This lecture assembles them, because a suite is more than a pile of tests and the difference is what a team actually experiences.
Decide what goes in it.
Not everything. A regression suite protects the journeys that cost money when they break: sign in, the main create-and-view flow, checkout, and the permissions that keep users out of each other's data.
Anything cheaper to check elsewhere should be checked elsewhere. Validation rules, formatting, date maths: unit tests, or API tests with Playwright's request fixture. Putting them in the browser makes the suite slow without making it safer.