Previously, this functionality was restricted to a curated selection of 360-degree videos. Android users have been able to watch any video in VR for some time and now iPhone users have the same freedom.
To enable Cardboard mode, tap the screen once the video starts playing and then tap on the three dots in the top right corner. This brings up an overlay that lets you turn on Cardboard by tapping the icon.
Google hosts its annual IO event this week and is expected to announce Android VR.