They aren't making a chip from scratch though--they've licensed an ISA and built a derivative design customized to their needs. This isn't to knock the work Apple has done--it's not trivial and clearly not everyone else is able or willing to do it. But going from absolute zero and inventing an entire new ISA, all of the tooling for it, etc. is an order of magnitude more work for both Apple and all of its developers. They made a smart move to use ARM's ISA and liberal licensing that allows them to build on top of it and make exactly the hardware they need.
This isn't a direct response per se, but your comments made me think of some relevant background.
Apple has been deeply involved with ARM almost since the beginning. Allegedly, the acronym "ARM" was changed from "Acorn RISC Machine" to "Advanced RISC Machine" at the behest of Apple, and their engineers seem to have been involved soon after the first ARM chip was created for internal use in Acorn's computers, making modifications to the chip and ISA to make it suitable for the Newton, their combined efforts creating the first commercially released ARM chip, the ARM6.
More recently, Apple has done a lot of work with LLVM. They weren't the original authors, but they've effectively created a lot of their own tooling.
All this to say, while they did license ARM, and they did start with someone else's tooling, they were so deeply involved in the origin/growth of both I think you may be underselling their involvement/work. If they didn't already have such deep historical ties to ARM, I suspect they would have seriously considered making their own architecture.
For all intents and purposes, they are. None of Apple's SoCs since the A6 (2012) have been based on ARM's Cortex-A cores; the CPU design is fully in-house at this point.
Yes, but the M1 is nothing like a new product. As you indicated, Apple's first custom SoC was released in 2010, 11 years ago. They have >10 years of experience shipping SoCs for Apple products. The M1 family can be viewed, to some extent, as an extension of the work on the Ax chips, which likely builds on their experience customizing chips for the iPod family.
The ISA is roughly equivalent to an API. It specifies how to talk to the chip but does not define how it is implemented. Apple has done a lot of custom design of their chips to optimize them for use with Mac OS and Mac software. This is not just Apple copying a chip design.
