The U.S. cannot regulate foreign companies doing business in foreign territories. But it can regulate US companies doing business overseas, and it can regulate foreign companies that do business in the US.
Even though most semiconductor manufacturing is done in China, it turns out that a mobile network (and all electronics, really) require lots of different kinds of chips from lots of different companies, and it’s very difficult to build a finished product without using American chip designs. It also turns out to be very difficult to be a globally competitive company without the US as a customer.
Intel, AMD, nVidia, Texas Instruments, Micron, Broadcom, Qualcomm, Cirrus Logic, Analog Devices, Microchip, Atmel, Conexant, and many others are all American semiconductor companies with a stake in the telecommunications market. If the US places an embargo on Huawei, it could impose fines and other punitive measures on US companies that sell to them, and can forbid Huawei’s products from being used in any security-sensitive American infrastructure.
Aside from US companies, the only other nation with as many intellectual property holdings as the US is Japan, and they’re not exactly friendly to Huawei either. Although they don’t call out Huawei specifically, they have banned all telecom equipment that might pose a national security threat (i.e. Chinese companies). Japan is also working to be included in the “Five Eyes” intelligence alliance, so it wouldn’t be very difficult to get Japan to go along with a Huawei embargo.
So while the US cannot directly enjoin foreign companies from doing business with other foreign countries, it does have the economic clout to get most of the western world on board with such a measure.