You can quickly compare your app/website with the designs as an overlay, using the Pop Out feature.
Click on the Pop Out button on the bottom left or simply use the shortcut ⌘O on the Mac app or Alt+O on the Windows app.
💁♀️ Pop Out is only available on desktop apps. But one of our lovely crewmates has just built a Chrome extension to make the Pop Out feature available on the Web app as well. You can also try this: Zeplin Popout Tool
Snap to simulator
On the Mac app, you can click the Phone icon on the bottom to directly snap the Pop Out to simulator in Xcode.
☝️ To be able to use the Snap to simulator feature, Zeplin's Mac app needs to have Screen Recording permission on your Mac.