This piece provides a fair bit of insight:

> Apple-TSMC: The Partnership That Built Modern Semiconductors

In 2013, TSMC made a $10 billion bet on a single customer. Morris Chang committed to building 20nm capacity with uncertain economics on the promise that Apple would fill those fabs. “I bet the company, but I didn’t think I would lose,” Chang later said. He was right. Apple’s A8 chip launched in 2014, and TSMC never looked back.

https://newsletter.semianalysis.com/p/apple-tsmc-the-partner...