Is there somewhere I can find how that list was generated?

While this is a Privacy and Security setting issue - at least tangentially - my question isn't about security so much as how this data is being stored/retrieved. I navigated to an entirely new site the other day and it requested a username and password. When the cursor was in the Username field, a set of proposed entries showed up. These were all (recognizable) first names - not emails. When I checked the Privacy and Security settings, I couldn't find any of the names in stored data. While this may be just cached entries for similar fields ("username" or "name" etc...), the list I saw was heavily curated. One of the names I don't ever recall even entering in any name field (and wouldn't have been used for any login) - but even if I had, the list that I saw was far from exhaustive and would only have been a very specific subset of any data I would have entered into any "name" or "username" field. Is there somewhere I can find how that list was generated? Thank you,