There are two main platforms in VR, one is mobile platform, like Google Cardboard, Samsung GearVR, the other is PC platform, like HTC Vive, Oculus, PS VR...
Unity officially supports the Oculus Rift, Google Carboard, Steam VR, Playstation VR, Gear VR, and the Microsoft Hololens.
Most platforms have their own support and sdk. Usually, you need to download the sdk as an extension firstly for unity.