However, "dead" does not mean "useless." There is a thriving preservation community dedicated to archiving (for WP7/8) and APPX files (for WP8.1/Windows 10 Mobile). The trick is learning how to download these application packages directly to your Windows PC before sideloading them onto your phone.
The Windows Phone ecosystem—encompassing Windows Phone 7, 8, and 8.1—is officially dead. Microsoft shut down the Windows Phone Store for consumers in December 2019 and for enterprises in early 2020. If you own a nostalgic Lumia, an HTC One (M8) for Windows, or a budget Blu Win HD, you cannot simply tap "Install" anymore. Download Windows Phone Apps -XAP APPX Files- to PC
"My PC says APPX is an unrecognized file type." Solution: APPX files are just ZIP containers. Rename .appx to .zip to extract the contents. To install, you need the Add-AppxPackage PowerShell cmdlet or the Windows Phone SDK. However, "dead" does not mean "useless