Samsung
Turn Bixby Launch on the Side Key On or Off auf Samsung
On older Samsung phones, a long press on the power button opened the shutdown menu. After Bixby was added to the system, Samsung reassigned that same long press to launch Bixby by default and renamed the button to Side key.
This is not a bug. It is a setting change buried in Advanced features, and it catches a lot of long-time Samsung users off guard because the muscle memory for turning off the phone no longer works the same way.
Where the setting lives
Go to Settings ➔ Advanced features ➔ Side key. There you get two options for the long press: Wake Bixby or Power off menu. Choosing Power off menu restores the classic behavior, so a long press brings up restart, power off, and emergency options again. Choosing Wake Bixby keeps the assistant on that gesture.
If you pick Wake Bixby, you still need a way to shut down the phone. On these devices that is done by pulling down the status bar and tapping the power icon, or by pressing the Side key and the volume down key together.
If Bixby keeps opening even after switching to Power off menu, check whether a second shortcut, like double-press, is still mapped to the assistant in the same Side key screen.