The shift has been years in the making, and Google says that 70 percent of sites shown in search results have already embraced mobile-first indexing. Between now and September, it will continue moving sites to mobile-first indexing when its systems recognize that they're ready.
Google started experimenting with a mobile-first index that ranked sites based on their phone-friendly pages in 2016, and the company made most of its searches mobile-first in 2018. By 2019, it was indexing new web domains as mobile-first by default. This is the natural next step.