Hiya. Author here. They wouldn't tell me. Asked a lot.

Presumably it's 1. personal data or some amalgamation of meta data thereof, 2. page rank, 3. rank brain

personal data and such would almost certainly be below page rank.

Not if that includes the geographical location of the query...

Personal data is almost certainly in the top two. Stating that, however, would raise privacy concerns from users.

> Personal data is almost certainly in the top two

Do you experience a drastic drop in result quality when you use a public computer?

I would guess relevance(how closely text on the page matches the query) and backlinks(how many popular websites link to the page).

Or if relevance doesn't count as signal maybe it's social sharing.

Google users feed Google with training data everyday, just by clicking on links and refining search queries. They basically tell Google what they were looking for. I have no idea how Google works, but I'd guess the user data plays a big role in rankinkg the search results.