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.