Do you know what Akinator is? 
I want to create similar thing, but instead of it guessing who the person is, I want it to generate a web app that have all the constraints in the questions.
What would be good questions for this kind of product?
Yes, I am familiar with Akinator. For a web app that generates constraints based on questions, you can ask questions that gather information about:

- User goals/objectives
- Available resources/technology
- User constraints (time, budget, scope)
- User preferences (design, functionality)
- Industry-specific requirements
- Target audience
- Security/privacy considerations

By asking these types of questions, you can gather the necessary information to generate a web app with constraints that meet the user's needs.