Most of the time, people have no way of knowing when you screenshot their stories. The functionality for such notifications isn't standard across social media apps. But always be respectful and use screenshots appropriately.
In most cases, no. Usually, there's no built-in notification feature that alerts someone when you take a screenshot of their stories.
Generally, people can't tell. Social media platforms don't typically have a direct way of notifying the creator of a story if someone screenshots it. However, some apps might have third-party add-ons or hacks that claim to do this, but they're not common or reliable.