“Oops! You’ve pointed app.domain.com to Bubble, but we don’t have an application associated with this domain yet!” So that more or less would answer your question.
You can’t set up subdomains within a single Bubble app. (ie. your index page cannot be www. and everything else is app.
It is good for your static (marketing) site to be different than your app. Primary reason: when you push changes to the static site it doesn’t impact the app (and vice versa).
Provided you do so, you have Bubble app 1 (marketing site), which is at domain.com. Then your app is at app.domain.com.