It deletes messages after viewing, but all that data is still stored on snapchats servers so it will live on even if you think its gone. Snapchat did randomly try to take a picture of me when it was off (I got a notification that it tried to without permission in the background, and if you google this issue so do a lot of other people) - but so does everything else, so what can you do besides not download it. People can see when you look at messages and can also see when you look at pictures once or twice. People can see when you add them as a friend, you have to add people as friends in order to send snaps.

There's a bunch of "stories" that are public ranging from individually curated stuff (the ever present influencers that hock beauty products) to mini news bites to complete clickbait trash (prank channels and the like). Ads are getting annoying. You don't have to use any of them, but I find the clickbait trash is fun if you have time to kill on the toilet or waiting for a bus or whatever.

Lots of people like the lenses that alter your face or add stuff to the foreground. It can be fun to play around with them with a friend. They probably use your face and data from that for machine learning training.