I haven't been following Oculus since I bought the first one on Kickstarter (which I had a lot of fun with); the Quest 2 has cameras to support AR or is that an add on? It looks like it on pictures, but there is little mention of it on most pages about it (at least at a quick look).
Yes, the Quest 1/2 and Rift S do "inside out" tracking. They have 4 cameras (5 on the Rift S) on each corner of the front face plate that tracks the headset and controller positions in 3D space (in addition to internal accelerometers and gyroscopes).
This is, for the most part, what enables the Quest to be fully wireless. Because there is no base station or fixed position camera, you're not limited to where you can use it, nor how big your play area can be. If you're in a new space, you just draw a guardian border around your play area and you're good to go.
The "passthru" feature mentioned above lets you see the camera feeds composited together in the headset display. It's very helpful for things like moving a chair out of the way or finding your controllers if you put on the headset without grabbing them. This new API allows developers access to this camera feed and to interact with its display for AR applications.
The first oculus headsets used camera base stations to track the headset and controllers, all of the recent ones have used cameras on the headsets itself to track everything, these can be used to see the real world, however since they are infrared cameras meant for tracking there is no colour and the resolution isn't as high as it should be for AR to be useful.
I had the first oculus, and it didn't have any base stations. It didn't track anything at all, in fact... it only detected rotational motion of your head, it didn't detect any other motion at all. There also were not any controllers.
The "dev kits" were freely sold to everyone and also bought by consumers, not just developers. Very different to sell-your-soul-NDA'd console devkits given out to select studios. "early adopter edition" would have been just as fitting.
Because even though they called it a dev kit, it was still a commercial product in many ways... there were a ton of games released for it, and a whole community of users who were not oculus developers. I got one, and I never wrote any software for it nor had any real plans to. I got it purely as a consumer.
