How to build website builder?

I'm thinking of building something like Squarespace or Cargo.site.
I'm mainly a front-end developer, focused on Vue JS, can use React. For back-end mainly use Laravel and Feathers JS, usually uses MongoDB and Postgres.

Where should I start? Any code example or article to look at?

My biggest questions are these:

1. How does the website data made by users should look like? (Data structures)
2. How to present that data as a website that site visitor can see?
3. How to make the site templates?
4. How to allow users to connect their own domain or make subdomains?