If you are buying an admission ticket (which means you don't already have a ticket which includes PI admission) you will need to stop at the ticket booths (which are where they've always been) and buy an admission ticket before entering PI if you plan on going to the clubs.
The streets are open. You don't need to buy a ticket unless you want to enter clubs. You can't buy a ticket at the clubs, you must buy them at the ticket booths. If you already have ticket media that include admission to PI (UPH, Park Hopper Plus, PI annual or Premium Annual) you just bring it up to a club for admission; it isn't necessary to stop at the ticket booths..
You will present your ticket at the first club you go into. They'll scan it, check your ID if they need to, and give you a wristband. The last time I was there, once you had a wristband you could use the wristband to get into other clubs and they wouldn't have to scan your ticket again. When they first started this method of entry they'd want to scan your ticket again at any new club you entered, and that would result in long lines at some clubs and cause anger in those who just went outside to smoke.