The new app platform happened maybe a couple years before the Nokia acquisition. I don’t remember the whole timeline but don’t forget Steven Elop (who had been the business head of Office) left Microsoft to become the Nokia CEO, had the company dump their own mobile platform for Windows Phone, then when that failed closed to deal for Microsoft to buy their cratered mobile business.

In the interim, Microsoft built quite a few phones mostly with Samsung, HTC, and Huawei, but also oddball partners like Dell.