: You must use a PC with the Windows Phone 8.1 SDK installed. Connect your phone via USB and use the "Application Deployment" tool to select and deploy your .xap file.
: You can often just copy .appxbundle files to your phone's storage and open them using the File Explorer app to install. windows phone xap archive full
Example extraction (conceptual):
: The Windows Phone 10 App archive Discord server hosts over 1,000 verified apps and games. How to Install (Sideload) These Files : You must use a PC with the Windows Phone 8