As Cal-Pie says, once you scan the bar codes from UCT into your app (which you can do as soon as you get the email from UCT, which for me took about one minute from purchase time), you go straight to any ticket gate at the park you want to enter. On first entry, they will scan the ticket from the app, then issue each person a paper ticket which you can use for claiming your fast passes through MP and to get back into the parks each day. They also take each person’s photo at that time, which is used to verify that the same person is using the paper ticket for park entry from then on.
Tip: Make sure to ”name” each ticket correctly as you scan each bar code in from UCT (the app initially defaulted to naming them all my name since I bought the tickets from UCT, but you can rename each one as you scan it in; however, you must rename right when you scan in the ticket or the option to rename disappears). Then when you get to the gate for the first time, scan the correctly named ticket, then have that person go through the turnstile and get their photo taken. This way the right ticket is named correctly in the app, which makes managing FPs for multiple people so much easier.
Last tip: Take a photo of the back of each paper ticket once they are issued on first gate entry. This helps tremendously if a ticket is lost in the parks and needs to be replaced. Also a photo of the ticket can be used to scan in for a FP if needed (app goes down, for example, which happened to us). I text the photos of all of the tickets to DH and DDs so that if we are in different parks and someone wants to use someone else’s FP, they can scan that person’s tickets at the FP machine from the photo (example: we booked 4 FPs for GOTG, but two of us are at DL and don’t want to ride it; the other 2 who are in DCA can each ride it twice, once with their own FP and once with ours; they can scan the FPs from the app or use their paper tickets to scan in for their own FPs, but they will need our paper tickets, the photos of our tickets or our FPs from the app to use our FPs; if the app goes down (and it does!), having the photo of the tickets can save the day and lots of steps trying to deliver paper tickets to them in another park; hope this makes sense).