Among the cactuses in the desert of Arizona, just outside Phoenix, an extraordinary collection of buildings is emerging that will shape the future of the global economy and the world.

The hum of further construction is creating not just a factory for the world's most advanced semiconductors. Eventually, it will mass produce the most advanced chips in the world. This work is being done in the US for the first time, with the Taiwanese company behind it pledging to spend billions more here in a move aimed at heading off the threat of tariffs on imported chips.

It is, in my view, the most important factory in the world, and it's being built by a company you may not have heard of: TSMC, Taiwan Semiconductor Manufacturing Company. It makes 90% of the world's advanced semiconductors. Until now they were all made on the island of Taiwan, which is 100 miles east of the Chinese mainland. The Apple chip in your iPhone, the Nvidia chips powering your ChatGPT queries, the chips in your laptop or computer network, all are made by TSMC.

Its Arizona facility "Fab 21" is closely guarded. Blank paper or personal devices are not allowed in case designs are leaked. It houses some of the most important intellectual property in the world, and the process to make these chips is one of the most complicated and intensive in global manufacturing.

They're hugely protective of the secrets that lie within. Important customers, such as Apple and Nvidia, trust this company to safeguard their designs for future products.

But after months of asking, TSMC let the BBC in to look at the partial transfer of what some argue is the most critical, expensive, complex and important manufacturing in the world.

President Trump certainly seems to think so. He often mentions the factory in passing. "TSMC is the biggest there is," he has said. "We gradually lost the chip business, and now it's almost exclusively in Taiwan. They stole it from us." This is one of the US president's regular refrains.

TSMC's recent decision to expand its investments in the US by a further $100bn (£75bn) is something Trump attributes to his threats of tariffs on Taiwan and on the global semiconductor business.

The expansion of the Arizona facility, which was announced in March is, he believes, the poster child for his economic policies - in particular the encouragement of foreign companies to relocate factories to the US to avoid hefty tariffs.

China is also watching developments carefully. Taiwan's chip-making prowess has been part of what its government has called its "Silicon Shield", against a much-feared invasion. While the original strategy was to make Taiwan indispensable in this area of critical technology, the pandemic supply chain difficulties changed the calculus because........

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