With the influx of people from the Quest 2, many are asking about how game purchases work with the Rift and Quest. Things are a bit more complex than you might expect.
Hardware & OS
The Rift CV1 and Rift S (I'll just call them Rift) are PC headsets. They connect with a cable to a Windows PC.
The Quest 1 and 2 (I'll just call them Quest) are stand alone headsets. This means they have their own computer built in and run games directly, no PC needed. They are Android based and use mobile phone chips. Quest 1 and Quest 2 are compatible for games and they use the same store.
The Go is also a stand alone headset. It runs Android on a mobile chip like the Quest, but is much weaker. It is also a 3DOF headset, which means it can't track movement, only rotation.
Software made for the Rift can't work on the Quest or Go directly, they aren't compatible. Likewise, Quest and Go software can't work on the Rift. It's like trying to play a PC game on a mobile phone, it's the wrong hardware and OS. If a developer wants to support Rift and Quest or Go they need to build their game for both platforms. How easy that is depends on the game, getting a Rift game ported to the Quest can require heavy optimisation, the Quest is just far weaker than a good gaming PC.
There are 4 Oculus stores: Rift, Go, GearVR and Quest.Rift Store
The Rift Store has windows based PC games. You can buy from the Rift Store using the Oculus website, the Oculus desktop app, the Oculus mobile app or in VR using the Rift.https://www.oculus.com/experiences/rift/Quest Store
The Quest Store has Android based stand alone games. You can buy from the Quest store using the Oculus website, the Oculus mobile app or in VR using the Quest.https://www.oculus.com/experiences/quest/Go Store
The Go Store has Android based stand alone games. You can buy from the Go store using the Oculus Website, the Oculus mobile app or in VR using the Go.https://www.oculus.com/experiences/go/GearVR Store
As far as I know, the GearVR store is the same as the Go store. Every Go game I've looked at says it is GearVR compatible too.https://www.oculus.com/experiences/gear-vr/
Third Party Stores
You need to enable Unknown Sources in your Rift settings to play games from third party stores.Steam
Steam sells VR games made for PC only. They don't sell Android based games that would run on the Quest or Go. The Rift can run any game from Steam that is made for SteamVR (like HalfLife Alyx), or some games include native Rift support.Epic Game Store
Same as Steam, the Epic store only sells PC games. If they have VR support using SteamVR or native Oculus support, they will run on the Rift.
Cross buy is when a game is available on both stores, and buying on one gives you a free copy on the other. For example, Audica is cross buy. If you buy it on the Rift store, it will become free on the Quest store. It works the other way too, buy on Quest and get it for free on Rift.
Developers choose if their game is cross buy. Some do (Audica, Pistol Whip, Vader Immortal, etc). Others don't (Beat Saber, Virtual Desktop, etc).
If a game isn't cross buy and you want it on both platforms, you have to buy it twice.
Games that are cross buy between the Rift and Quest: https://www.oculus.com/experiences/quest/section/2335732183153590
The Go has a limited selection of games that can be played on the Quest. They won't appear in your Quest library, you need to change the library in your Quest user interface from Quest to Go and they will be visible.https://support.oculus.com/684583725353553/
Steam and Epic Game Store aren't cross buy with Oculus. If you buy Beat Saber on Steam, you can run it on the Rift, but you don't get it on the Oculus store, it's still only on Steam. While Audica is cross buy for Rift and Quest, if you buy the Rift version of Audica on Steam you don't get a Quest version too, only games bought on an Oculus store count for cross buy.
The Link cable changes some of the above, to a degree.
The Link cable lets a Quest connect to a PC. It effectively turns the Quest into a Rift. This means it now runs PC Rift games. It can play anything a Rift can, including games from Steam or Epic Game Store. It works by streaming video over the USB Link cable to the Quest, and sending back the tracking data. At least for now it's a lower quality option than running natively, its like watching a game streamed on Twitch, there can be video compression artifacts and latency. Some people notice the issues more than others.
Any USB cable (with USB-C on one end) should work for Link, but there's an official Link cable which is rather expensive, but it's an active 5m fibre-optic cable, which is always expensive. Getting a 5m cheaper cable that works is tricky, most USB3 cables max out at 3m.
Virtual Desktop works like Link, but uses Wifi instead of a USB cable. It also streams Rift games to the Quest. You need to buy the Quest version of Virtual Desktop if you want to stream PC games to the Quest. The PC version of Virtual Desktop is different.
I've never used Virtual Desktop, so I can't say too much about how well it works.
Revive is a third party program that lets other VR headsets (Index, Vive, Reverb, etc) impersonate a Rift and play games from the Rift store. This isn't guaranteed to work for all games, some have issues and don't work. But it does at least give a chance to not waste your whole library if you move to a non-Oculus headset.