I want to donwload the asset as an .unitypackage file.
The reason is that I want to reuse it for different projects. I don’t want to download it again every time.
When you download something from the asset store it is saved as a .unitypackage at this location:
C:\Users[User]\AppData\Roaming\Unity\Asset Store-5.x
Most likely you download it as .unitypackage already (you for sure do if you download them from within Unity Editor’s Asset Store window).
As I understood from your comment you use Unity Editor’s Asset Store window to download assets, if not then let me know how you get them in the comment section below.
Now to answer your question - yes you can look for them in that folder, you can also use a Download Manager button within Unity Editor’s Asset Store window to browse all downloaded assets:
there you’ll also be able to re-download, update and import assets to your project with respective buttons:
You can move those assets from that folder if you’ll want to but in that case Download Manager won’t see them (you will still be able to import in other ways, which, considering your question, you already know how to do), also you can create symlink of whole folder if you’ll have space issues so that you’ll not need to bother with relocating packages and Download Manager will work perfectly fine.
When you download something from the asset store on macOS it is saved as a .unitypackage at this location: /Users/[User]/Library/Unity/Asset Store-5.x