Samsung
Quick Launch Camera via Side Key auf Samsung
When a double press on the Side key stops opening the Camera app from the lock screen, the action assigned to that key has usually been changed or switched off.
Samsung controls this shortcut through the Side key settings inside Advanced features. There you pick what a double press does: launch the Camera, open a chosen app, or trigger Bixby, depending on your model and software.
To re-enable the shortcut, go to Settings ➔ Advanced features ➔ Side key ➔ Quick launch camera.
The double press works both on the lock screen and on the home screen once selected. From a locked device, it opens the Camera directly without unlocking, so you can capture a shot before entering a PIN or pattern. Photos and videos taken this way save normally, but the Gallery itself still requires authentication.
Only one action can be tied to the double press at a time. If Bixby or another app is currently assigned, selecting Quick launch camera replaces it, and the previous shortcut stops responding to that gesture.
A single press of the Side key is unrelated to this feature; it only wakes the screen or, when held, brings up the power menu.