With iOS 12, Apple has given a big update to supported iPad models. all of iPhone X gestures now work on iPad too. And that changes the way you do things on your iPad, like opening the control center is now different.
On an iPad running iOS 12, you won’t get control center alongside the app switcher. Instead, you have to swipe down from the top-right corner of the screen to open control center on iOS 12.
It looks like this: