Where MS Partner Center stores *.appxupload packages?

The Microsoft Partner Center does not store the *.appxupload packages directly. When you upload an *.appxupload package to the Microsoft Partner Center, it is stored in the Microsoft Store submission system, which is separate from the Partner Center.

Once you submit an app package (*.appxupload) through the Partner Center, it goes through a validation and certification process. If the submission is successful, the app package is stored in the Microsoft Store's backend systems.

When a user installs your app from the Microsoft Store, the package is delivered from Microsoft's distribution infrastructure, not directly from the Partner Center.

In summary, while you upload the *.appxupload package through the Partner Center, it is stored and distributed through the Microsoft Store's systems rather than being stored within the Partner Center itself.