Does instagram show screenshots - what's the real answer?

yep that's it. screen recording one of those view once photos counts the same as a screenshot, people sometimes think that gets round it. vanish mode chats are the other one, screenshotting those tells the other person too. a normal dm conversation doesn't.

No for posts, stories and reels. The one exception is disappearing photos/videos sent in DMs (the view once ones) - screenshot those and the sender gets told.