Samsung combined the Bixby button from the S10 Series and the Power button into the "Side Key". To change it, press and hold the "Side Key" and Volume Up and the Power Off menu should appear. Then tap Side Key Settings on the bottom of the screen and change the Button Hold option to "Power Off".
Hope that helps. I strongly recommend exploring the capabilities of the Bixby voice assistant though. 👌🏻
You could use Goodlock app and with Navstar module get an extra button on your navigation bar to turn off screen or take a screenshot or you can use Registar and add Google Voice Assistant just by tapping the back of your phone 2 or 3 times!!! It can also take screenshots and share them or open a pop-up window or open any app
