No need to download it, it's not possible. Simply create an account and then you are ready. All works directly in the browser.

After you are done with your project you can save it online in your account. I made mine only accessible for my own.

You can, of course, also download all objects you made to your computer as a backup if you like. But if you import them again in TinkerCad, the steps you made to create the object are lost. This means you only have the final result and can not go back the whole steps you did to create it. You can, of course, work further on it.

You can also import any OBJ file and adjust it the way I descripted it before.