User blog comment:MisterWoodhouse/Evolving with the Web: Keeping Google Happy/@comment-10637-20210114165554/@comment-24006128-20210114172411

Both get crawled, but mobile is first. They index both versions and compare them for differences.

The mobile, logged-out crawl, however, is the basis for rankings.

Sometime soon, they'll stop indexing desktop altogether, as mobile represents the vast majority of web traffic globally and Google wants to reward sites which present a good mobile experience, not only from a tech perspective, but also from a content perspective.