The most common way to turn off a phone screen and lock the device is to use the on-off button. That usually sits on the side of the phone and needs to be squeezed quite tightly. You need to do the same to turn the screen back on.
However, there's a quicker and more elegant way to do it - a simple double tap on the display, assuming the manufacturer supports this feature.
Some like Samsung do but others only go halfway. Google's Pixel phones, for example, can be brought to life with a double tap but can't be put back to sleep the same way.
This is where an app like Screen Off Widget comes in handy. A free app that works with Android 9 and higher, it can be used to place an invisible area covering up to nine tiles on the home screen.
This area then responds to the double-tap command to turn off the display and lock the device. Because the widget requires special rights as an operating aid, Android will initially warn you or at least ask you whether you really want to use it, but the notification can be ignored.
On some phones you can activate the screen and display the lock screen by simply lifting them up, like with Apple's "Lift to wake" feature.
Apple also offers "Tap to wake" for many iPhones. To activate the feature, go into Settings > Accessibility > Touch > Tap to Wake.
To lock the device, you will have to give it a double tap on the back however. That option can be activated under Accessibility > Touch > Back Tap or Accessibility > Touch > AssistiveTouch.