We are not providing Windows ARM64 images, but there are customers running BYOC instances with Windows ARM64 images provided by Azure. AppVeyor is able to create ARM64-based instances on Azure - you can see "Architecture" dropdown on Azure cloud settings in BYOC. Build agent can be run inside ARM instances too. Though we don't have instructions nor automation script to bring BYOC to Windows ARM instances. Those customers who did that were making based images manually (using their own tools).
Also, we could run Ubuntu ARM64 builds on AWS, for example this build.