It's Tim Cook's last day as Apple CEO. Investors are $4 trillion richer because of him.

Apple's ( AAPL ) Tim Cook is stepping down as CEO today, turning over the role to senior vice president of hardware engineering John Ternus.

Cook will stay on at Apple as its executive chair and "assist with certain aspects of the company, including engaging with policymakers around the world," the company said in a statement.

During his time as CEO, Cook oversaw the launch of a variety of new products, including the Apple Watch, AirPods, and, more recently, the MacBook Neo. He's also been instrumental in increasing Apple's Services segment revenue to more than $100 billion, the company's second-largest business behind the iPhone.

Steve Jobs hired Cook in 1998 and promoted him to executive vice president of worldwide sales in 2002, before appointing him to COO in 2005. He later took over as CEO from Jobs in 2011.

During that time, Cook leveraged his supply chain expertise to dramatically expand Apple's business. The company's market capitalization has grown from $350 billion to more than $4 trillion, a staggering 1,000% increase.

Yearly revenue quadrupled from $108 billion in 2011 to $416 billion in 2025 . Apple's iPhone revenue, in particular, skyrocketed under Cook's leadership, from $47.1 billion to $209.6 billion.

And the company's Apple Watch and AirPods are the world's most popular smartwatch and headphones.

Importantly, Cook pushed the company to transition from using Intel ( INTC ) chips to power its Mac line of products to its own custom silicon, which has helped boost the performance of its laptops and desktops, allowing them to provide dramatically better battery life that competing systems are only now catching up to.

All of that has contributed to Apple's massive stock gains since Cook took over. From August 2011 to Monday, Apple stock has climbed more than 2,000%, easily outpacing the S&P 500's ( ^GSPC ) greater than 500% gain.

John Ternus will be stepping into the position at a time of major transition for Apple. The company is working to bring its AI capabilities up to speed after falling behind competitors such as Microsoft ( MSFT ) and Google ( GOOG , GOOGL ).

It's now expected to launch an updated version of Siri powered by Google's Gemini AI models during its WWDC conference in June.

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Apple is also reportedly preparing its most dramatic iPhone change yet: a foldable model that the company is expected to debut shortly after Cook steps down in September.

Ternus will also launch Apple's rumored smart glasses, as well as any future AI-focused products meant to compete with OpenAI's planned AI-first device.

Email Daniel Howley at dhowley@yahoofinance.com. Follow him on Twitter at @DanielHowley .