Like YouTube for Android devices, version 12.03 of the iOS app adds play, pause, previous, next, and volume controls to the iPhone's lock screen whenever YouTube is casting to Chromecast devices, smart TVs, game consoles, and other living room devices. The same controls can also now be found in the iOS Control Center.
In addition, the update enables iPhone owners with a paired Apple Watch to access the same controls from their wrist. YouTube is available to download for free from the App Store. [Direct Link]