Everything you post is public. Even if you lock down your settings, your information can still be seen.
Locking down your friends list does not prevent your friends from showing up in the people you may know suggestions. The algorithm wants to connect you with people you potentially know in the hopes that friendships that were lost over the years can be rekindled. Because of that, locking down your friends list will not stop the algorithm from showing those friends to other people who may know them.
Some people have multiple accounts, for multiple reasons. If your account is public but someone is blocked, they can still see your account from another account. If you are posting content that you do not want seen, be aware that it can be seen from another account or a friend’s account.
Content is stored on servers, even if it is inferred that it disappears. Snapchat content is not completely gone when it disappears from your sight. It’s stored on servers and available for law enforcement if needed.
You can lock down every single setting, and privacy is very likely with settings locked down, but be aware that every piece of content that you produce or share may be visible in some way and may be brought out in an unintended way.