Apple and Samsung dominate the US phone market, and they’ve done so for years. Together with Google, they’ve shaped our sense of what a smartphone is and what it can do, pushing the boundaries of mobile photography, software, and processing power. But over the last few years, they’ve sat back, content to iterate rather than innovate — and in the interim, China’s tech giants have plowed ahead. Now a gulf is growing between the phones on sale in the US and those available in the rest of the world.