What happens when you close your MacBook Pro lid? Various questions.

These are such basic, simple questions I'm embarrassed to ask it after all these years. But I can't find definitive answesr in the support docs. And lately Apple Support has been extraordinarily unhelpful; I feel I know more than the advisors I speak with.

I have a new MBP M1 Pro. It's connected to an external monitor during the day. After 20 minutes away the screens go into screen saver mode. And later they goes dark. Starting from the time it goes into screen saver mode I need to use my TouchID to open it again. Why? Has it gone to sleep? Or is it sleeping when it goes dark? Or is it just saving the screen by going dark and not really sleeping at all?

I was thinking of closing the lid when I go to sleep, just to keep the screen from getting dusty. But I notice if I close the lid and open it up everything on my external monitor has moved to my main screen. Is there a way of getting everything back to where it was easily? If not, I'll just leave it open.

Thanks.