That's what the lock screen is for. To lock your phone when you are not actively using it. if you want to hide things from prying eyes, you have the secure folder option, or you can hide apps so that they can only be accessed if you search for them.
There's two ways other than your regular phone lock screen , you can use the Secure Folder to hold any private pictures that you don't want people seeing or you can lock an entire app where if someone does open it when they have access to your phone it opens like normal but they can't clear it unless they know how to because the lock prevents the app from being closed so you can catch the snooping person going through anything that you didn't give them permission to use or look at. The Secure Folder has the ability to have a lock which requires password or fingerprint to open it , you can move any app into it and it's a very good place to hide your pictures and videos you don't want others to see.
