Maybe it is something to do with Apple because I did it in Safari on the top line of the browser.
I normally search for stuff from the google home page, but I just searching from address bar, on Safari on an iPad, iSO 11.2.5, for the word firebrick, unquoted.
The top line presented was a list of ‘shopping’ results for building materials.
The second line, and first proper search result, was firebrick.co.uk.
It is not a search term I have used much, if ever, before, so I don’t think Google would have been giving me preferential treatment on basis of search history.
But, from their cookies, they do know I spend a lot of time on this website where firebricks (networking products) are widely discussed. So they could deduce I have at least a passing interest. Same may apply to other forum members too?