ISA is just about the instruction set that the silicon is designed to interpret. Aww compatibility does not imply any shared heritage in the silicon between apple and arm LTD cores. Just like AMD and Intel share no silicon design even though they are both sell processors implementing the AMD64 ISA.
(The silicon implementation of an isa is referred to as the microarchitecture btw)
Right, but the ARM ISA is covered by various patents, so you have to get a license for it. ARM has aggressively shut down any open efforts to make ARM compatible cores (without licenses).
(The silicon implementation of an isa is referred to as the microarchitecture btw)