This question is coming up a lot for those new to VR, and here are the simple answers.
In short, the answer to "do you need a computer or phone to use a Quest 2?" is yes and no. That's because that question can mean different things. I'll explain.
First of all, a brand new Quest 2 straight from the box needs to go through a setup process before it can be used. This setup process needs a phone with an app installed that will pair with the headset. Once this setup is done, the Quest can be used without the phone. In short, a phone is required for initial setup of a Quest headset, but is NOT required to use the Quest afterwards.
If you mean: "Can I set up the Quest 2 without a phone?" the answer is NO. The Quest 2 needs you to install an app, after which your phone will be used to complete the setup process (which involves creating an account.)
If you mean: "Do I still need the phone to use the Quest 2 after setup?" the answer is NO. Once the Quest 2 is setup, the headset can be used without the phone being involved. All you need is the Quest headset. You can pack it up and bring it somewhere, and it will work to play games without the phone being around. (There are some features that will only work with the app, but the headset will work and play games just fine by itself.)
If you mean: "Can I use a computer to setup the Quest 2?" the answer is NO. A computer cannot be used to set up the Quest. A phone with the app is needed.
If you mean: "Can I use the Quest 2 to play PC VR games?" the answer is YES. The Quest works like a games console, like a Nintendo Switch, or Sony PlayStation, or Microsoft Xbox, and does not require a computer. However, if you wish to use the Quest 2 to play SteamVR games instead of using it like a console, that can be done.
More info on playing PC-based VR content using the Quest
There is another, optional way to use the Quest, and that is by using a Link Cable or Airlink.
To use the Quest as a headset for PC-based VR content, you will need a Link Cable, or use wireless Airlink. They both do the same job, just with a cable (the Link Cable), or wireless (Airlink).
When using the Quest in this way, the PC runs the VR content, and the Quest is used to display it to the player.
If you don't plan to use the Quest to play SteamVR games or PC-based Oculus VR games, you can ignore this entirely.