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Like x86 and x64, ARM is a different processor (CPU) architecture. The ARM architecture is typically used to build CPUs for a mobile device, ARM64 is simply an extension or evolution of the ARM architecture that supports 64-bit processing. Devices built on the ARM64 architecture include desktop PCs, mobile devices, and some IoT Core devices (Rasperry Pi 2, Raspberry Pi 3, and DragonBoard). For example the Microsoft HoloLens 2 uses an ARM64 processor.
About the architectures, you can refer to this document.