I guess youll find less of complex/large applications built with a tool with specific purpose to be minimal and simple.

If anything, Flask is used as part of something larger, but its then not really complex, as that is not a goal for anyone sane, it would be a mistake. And noone is going to flaunt around their mistakes.

You should look for usable applications. And then you're looking at problem domain, will you find an audio software system built using Flask? I hope not. Will you find a "web content management system"? Yes there are some.