I have been thinking starting a blog on the topic considering how often this question keeps popping up. From a very high level overview.
1. Apple is partnering with TSMC. TSMC is now the leader in leading edge semiconductor manufacturing. A title that used to belong to Intel.
2. TSMC is now a generation ahead of Intel, meaning those thermal efficiency you see comes from using a better node. Nothing much to do with ARM or x86.
3. Both ARM and TSMC has dramatically change the Industry, you can now buy Designs / Blueprints from ARM, ( Or any other IP vendors such as Img PowerVR ) and Fab ( meaning producing them ) them with a Foundry.
4. TSMC is a Pure Play Foundry, meaning the Foundry does not produce their own chip and sell in the market to compete with its customers. A Non Pure Play Example would be Samsung or Intel, where Samsung produce their own Mobile SoC Exynos, and Intel with their x86 Chip. If you were Qualcomm producing your chip in Samsung's Fab, you are directly competing with them.
5. Apple now has the volume, or economy of scale to produce CPU themselves. Apple makes more silicon in unit volume than Intel per year.
So the inevitable Question:
Why Doesn't Intel go to TSMC then?
Intel would earn more margins by producing it themselves. Second being moving these designs take years, especially for Intel which have their own design tools in house. Intel cares about margins, and you can tell from their investor meetings.