I understand that products and services sold by US companies boost USD (vis-a-vis buying iPhones in your example), but why would, say, a Brit buying a Xiaomi ph

I understand that products and services sold by US companies boost USD (vis-a-vis buying iPhones in your example), but why would, say, a Brit buying a Xiaomi phone using USD boost the USD? The USD holding time would be rather short, I would imagine.