The expansion of the loophole for tariff-free shipments of goods nearly a decade ago gave rise to Temu, Shein and other low-cost online retailers offering items straight from Chinese factories at unfathomable discounts. It also unleashed something else — a cascade of billions of dollars of digital advertising that provided a windfall for Meta, Alphabet and other technology industry giants. Temu and Shein, jockeying for the attention of American shoppers, blanketed seemingly every inch of the internet with their ads. In the past two years, only…