How does the exit modal determine whether to show up?
The exit modal shows up once every 90 days per customer using a cookie.
For testing purposes, you can always delete the cookie manually (It's called viewedOuibounceModal) and refresh the page in order to get the modal to show up.
As far as the exit mechanism itself, it detects the mouse cursor moving towards the top of the screen outside of the browser. That's how it knows that the customer is about to exit.
Because it relies upon mouse movement, it doesn't show up on mobile or other touch devices.