ARM will license their CPU design to anyone, and Apple didn't just take the ARM CPU as is, they are using the ARM Instruction set, as well as design elements from the ARM Cortex CPU. Then then added a whole crapload of other things, such as their Neural Engine, a Storage Controller, Secure Enclave, etc etc... and out came the A4, A5, A6... etc.. etc.. and so the whole thing in it's entirety is what Apple calls "Apple Silicon". It actually would be incorrect to call it an ARM CPU, because it's technically not. I can't say I like the name either, but you can't really call it an ARM CPU either, I think too many people are confused over that part, it's not a pure ARM CPU. It has the Instruction Set, and some Designs taken from the ARM Cortex CPU, but it's Apple's own CPU designed from the ground up.