Turn both of the options on to turn the screen off.
Hope this helps.
Edit: Sleep mode can be activated as part of your schedule in the Health app, but it doesn’t have to be; it can also be used without the Health app scheduled bed/wake times via a Shortcuts automation, or manually, the same as any other focus mode.
As others have said, Sleep mode is the way to go with the iphone. It will also put the watch into Sleep mode which turns off the always on screen and requires a tap on the screen to show the time and whether an alarm is set in a subtle green colour.