As Flask does not enforce a project structure, you've quite a lot of freedom to try out what best suits you. Large one-file projects work, Django-like structured projects work, flat architectures also work; the possibilities are many! Because of this, many projects emerge with their own suggested architecture; these projects are called boilerplates or skeletons. They focus on giving you a recipe to quickly start a new Flask project, taking advantage of their suggested way of organizing the code.

If you plan to create a large web application with Flask, you're strongly advised to take a look at at least one of these projects because they've probably already faced a few problems you could face and have come up with a solution: