Mobile-first indexing
Since 2023, Google indexes only the mobile version of sites: content the smartphone does not render does not exist for the index, regardless of how rich the desktop version may be.
In practice
Classic pitfalls: folded content absent from the mobile DOM, truncated menus that break internal linking, heavy images not resized for small screens. The control reflex: audit with a mobile user-agent and compare the HTML served to each format. A well-implemented responsive design neutralizes the issue by construction.