Mostly, if Instagram's built - in screenshot detection is enabled for the account (which is often the case for private accounts), then yes, the person who posted the story can be informed that you took a screenshot without the use of third - party apps. But for public accounts, this may not always be the case as Instagram's policies regarding screenshot notifications for public accounts can vary by region and other factors.
Yes, Instagram has its own system to detect screenshots for some types of accounts. If you screenshot an Insta story from an account where this feature is enabled (such as many private accounts), the owner of the story will get a notification without any third - party interference. However, the implementation of this feature can be a bit inconsistent. For example, sometimes there can be glitches in the system and the notification might not be sent even though a screenshot was taken, or vice versa.