If people are signed into a Google account when they’re using the search engine, all the activity is recorded by default. To be fair, Google does allow users to stop this, and it does allow past history to be deleted. There’s a good article on all this here.
However, there’s always been some debate about how much past activity affects the results people see in Google search. Obviously this is important to us in SEO: if the influence is strong, we’re going to see results which are ‘better’ for our own companies than those seen by customers. That’s why independent monitoring of ranking is important.
The view from privacy-first search engine Duck Duck Go is that “editorialised results are informed by the personal information Google has on you (like your search, browsing, and purchase history), and puts you in a bubble based on what Google’s algorithms think you’re most likely to click on.” Their view is worth reading.
For its part, Google says that “personalisation doesn’t happen often and generally doesn’t dramatically change search results from one person to another”. My experience is that it does happen, but that it’s not significant in B2B marketing. We should be aware though that when it does have an impact, it’s likely to favour our own company when we’re looking, and perhaps favour customers’ existing suppliers when they’re looking.