On iPhone you can swipe left on the Lock Screen, or press the icon on the Lock Screen, or you can pick it from a menu you pull down from the top of the screen,

On iPhone you can swipe left on the Lock Screen, or press the icon on the Lock Screen, or you can pick it from a menu you pull down from the top of the screen, or find the app via icon, search, full app list. Maybe other ways I don’t use or know. I would assume that it’s similar on android, access the camera from lock screen or app. I’d probably try the iPhone style swipe the lock screen if I didn’t see a shutter or camera looking icon, otherwise I’d use search (if I could find that) to find the app.