re: How do you use Global Entry?

If you enter your number when making a booking, it is far more likely that you will get TSA PreCheck (though not guaranteed).

Upon re-entering the country, GE allows you to use self-service kiosks at Customs: you answer the questions, scan your passport, and put your palm on the glass fingerprint scanner. The kiosk takes your photo & prints a paper receipt, which you hand to the CBP officer on the way out. IOW, you don’t stand in the long, snaking lines with everyone else. Though at some airports, there is more demand than there are kiosks, or kiosks are broken, etc. In any event, it’s still quite a bit faster at very busy airports than standing in line & talking to an officer.