The startup module can be used to decide what form to show at startup. That’s better than guessing what you should show only to realize later that you needed to show another form, for example a notification to the user that they need to check the confirmation email or that they have won the lottery.
Once that form has been shown, each form will decide what to do as usual. You can have all forms sharing the same logic imported from a module, but I think the startup module is out of the picture now.