Only you can see the posts you've saved. When you save someone's post, they're not able to tell that you've saved it.

Private collections you've already created can't be shared with others. A collaborative collection can't be made private after it's created. When you add a private account's post to a shared collection, only people who follow that account will be able to see the post. The person who uploaded the post won't be notified.

Going public on Instagram is a matter of your account privacy settings. Unlike Facebook, you won't be able to pick and choose the privacy settings for each Instagram post. Instead, you'll have to make your entire account viewable to the public.