Why was Apple spending $35 mn/yr flying techies to China in pre-COVID era?

Apple was spending $35 million a year booking 50 daily business-class seats to fly its engineers from San Francisco to Shanghai on United Airlines before Covid-19 pandemic erupted. This was done to work side-by-side with partners like Foxconn and make sure various Apple devices were worked on in a collaborative, on-site process. CEO Tim Cook called it a "hand-in-glove partnership".