Since the official store is defunct, you must source files from reliable third-party repositories. Microsoft Learn WUT (Windows Universal Tool)

To move these files from your computer to your phone, you'll need specific deployment tools: Just released: Windows 10 Application Deployment tool

This guide provides legitimate and technical methods to acquire .xap and .appx files for offline backup or manual installation.

Several community-driven archives provide .xap files for sideloading. Examples include: