Apple got rid of the iPhone's home button and assigned its long-press action (Siri) to the power button. Since it now did more than sleep and wake the screen and bring up the power off dialog, they renamed it the "Side button." Well, Samsung just removed the Bixby key, and they're taking the same approach.
People complained about the dedicated Bixby key on the last few generations, so Samsung ditched it with the Galaxy S20. But they're not quite ready to ditch Bixby itself, so they made long-pressing the power button on the S20 bring up the digital assistant. But if you don't use Bixby, you'll be better off if the newly-renamed "side key" brought up the "Power Off, Restart, Emergency Mode" menu like it used to.
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Open your phone's Settings app and scroll down a bit. Select "Advanced features," then at the top of the next page, tap "Side key." Here, change the "Press and hold" setting to "Power off menu."
Now, when you press and hold the button on the side of your phone, your power menu will come up instantly. No traces of Bixby anywhere!