Search results come in many shapes and forms, for this audit only organic results and advertisements will be considered, leaving instant answers and other unique types out of scope (but based on what I shared in the audit can be made accessible).
Different versions of search results
The tooltip that provides information on how ads are privacy-protected is currently inaccessible through keyboard navigation.
The “Ad” tooltip
Tooltips, especially those containing interactive elements like the "more info" link in the Ad tooltip, are notoriously difficult to implement correctly and accessibly. As the content for the tooltip is the same for all ads, I suggest a more robust and accessible approach would be to make the labels buttons and display the content inside a modal dialog, rather than attempting to make the tooltip accessible.
Additionally, the report ad functionality could be moved inside this new modal dialog, which would save a tab stop and further streamline the user experience.
The proposed modal dialog instead of tooltip
Be sure to check my recommendations about how to make modal dialogs accessible in 3.4 Modal dialogs.
To enhance keyboard usability even further, the number of superfluous tab stops should ideally be zero. Currently, a regular search result has three tab stops, including the favicon, the URL of the result, and the title of the result. Since the title and the URL link to the same resource, one of those could be removed from the tab order to save a lot of keypresses. The same thing could be done with the ad results as well. Fixing these issues is as simple as adding tabindex="-1" to them.
However, there is also a parsing bug with ads, which makes multiple lines in the description separate links. This makes tabbing through them pretty arduous.