Because the official store is no longer active, users must use specific PC-based tools to install these archived files:
This report explores the technical nature of the .xap file format, the collapse of the official distribution channel, the rise of "grey market" archiving, and the legal and ethical quagmires surrounding the preservation of defunct digital ecosystems. windows phone xap archive
If you have a physical device running Windows Phone 8 or 8.1, you can sideload apps without complex tools. Because the official store is no longer active,
Many apps were removed from the store even before it closed; archives often contain versions of software that haven't been seen in over a decade. Mechanics of Survival: Sideloading and Deployment Mechanics of Survival: Sideloading and Deployment Or so
Or so it seemed.
We must address the elephant in the room. Is downloading from a piracy?