Unlike Windows 10 and 11, Microsoft never released a public ISO or retail version of Windows 8.1 for ARM64. Windows RT (the ARM version of Windows 8) was locked down tight. It was restricted to specific devices (Surface RT, Lumia) and signed firmware. You cannot simply download an ISO and boot it from a USB on a generic ARM device.
If you are looking to install a Windows OS on modern ARM64 hardware (such as Apple Silicon via virtualization or Snapdragon-based PCs), the standard practice is to use . windows 81 arm64 iso install
: Windows 8.1 RT is a 32-bit architecture. Modern ARM64 hardware (like Apple Silicon or Snapdragon X Elite) requires a 64-bit OS to run natively. Windows 8.1 does not support these 64-bit ARM processors Virtualization Constraints Unlike Windows 10 and 11, Microsoft never released