Before Bixby, the Samsung device could be turned off using the familiar on/off button, but pressing and holding the power button (Samsung now calls it a function key) will start Bixby. Turning off is now done via the status bar or by pressing the power and silent key combination.

If you want the usual way of turning off the Samsung device, you can deactivate Bixby on the on/off button, which will open the menu for turning off, restarting or emergency mode as usual.

Android 10 (One UI 2.0)
Step 1:  Open the  Settings
Step 2:  Tap on  Advanced features
Step 3:  Tap on  Side key
Step 4:  Tap on  Power off menu
  1. Open the Settings
  2. Tap on Advanced features
  3. Tap on Side key
  4. Tap on Power off menu

Samsung Instructions