Thank you for your response.
Most traffic comes from search engines, so I'm guessing that the site does well for topical searches and thus for keywords. From my experience, the site tends to land on the first page of search results, usually in the first position. I don't know what "1-3-5 position" means (sorry!), so I can't comment on that.
I'm told that, given the topic area and the traffic, the income is surprisingly good. This may be a result of the one particular client relationship, which suggests that the monetization is not particularly "safe" or "solid".
The traffic is organic, but has taken more than a decade to grow to the current level. Since the site is educational, any advertising at all is generally regarded (within the educational "industry") as a bad thing, so it would likely be difficult to engage in further monetization without a resulting "hit" in traffic.