if you bought it via vercel, then it's here inside the "domains" tab (see attached). Click there and you can manageyour domains. If you are in any project, you can directly add it to your project domains (see attached as well)

I had 2 teams. Domain was bought under Team A. I transfered the Project to Team B but I didn't transfer the domain because I thought it was tied to my account not the project. I deleted the project. I have no domains. In Team B, and I can't add the domain because it says one of my projects is already using it: but I only have one project: So I don't know what to do

Then you can only go to https://vercel.community/ and get help directly with vercel or check the links in #vercel-help