It replaces the whole CMS backend. You do not have to develop anything in Wappler to manage the content. Imagine a Wordpress blog. Strapi is what you get once you login into the admin area. You get a dashboard to manage your content, but you also get an out-of-the-box solution to create posts, categories, drafts, taxonomies, etc
Exactly. You would only develop in Wappler the frontend functionality to consume the content. In the Worpress example you only need to develop the views for the blog, single-post, categories, etc. You would consume from Wappler the strapi content via API (REST at the moment, GraphQL hopefully some day :))
You can use the same database you are using for your project or you could use other databases. You can even use a MongoDB database which is currently not supported by Wappler because you will consume the info via API.