How does an SEO company track user engagement?
Mainly in Google Analytics, by reading what visitors from search do after they land: whether the visit counts as engaged, how long the page held their attention, how far they scrolled, whether they went on to another page or took an action, and whether they came back. Read page by page, these figures show whether a page that ranks also serves the people it attracts. They are evidence about your visitors, not a ranking score, and a company should treat them that way.
What Google Analytics counts
Google Analytics builds its engagement figures on the engaged session. Its help page on engagement rate and bounce rate defines one as a session that lasts longer than ten seconds, includes a key event, or has at least two page or screen views; the engagement rate is the share of sessions that meet any of those conditions, and the bounce rate is the share that meet none. An SEO company reads these for organic search visits, landing page by landing page, rather than for the site as a whole.
Scrolling is measured automatically if the site uses enhanced measurement. Google’s enhanced measurement documentation says the scroll event fires “the first time a user reaches the bottom of each page (i.e., when a 90% vertical depth becomes visible).” That is a single reached-the-end signal. To see where readers stop on a long page, a company has to add its own scroll depth events at earlier points, and should tell you that it has. The same feature records clicks on links that lead to other websites, which shows when a page sends people elsewhere.
Returning visitors complete the picture: a page that brings people back is doing something the first visit alone cannot show.
What the figures do and do not show
Engagement figures describe what happened on your site. A short visit can be a success: a visitor who wanted your phone number and found it in five seconds is not a failure. A long visit can be a problem: a reader may have struggled to find the answer. So the figures are read together and against the purpose of the page. A contact page, a long guide, and a product page each have their own sign of success.
On ranking, Google’s description of how it ranks results states: “We also use aggregated and anonymized interaction data to assess whether search results are relevant to queries.” That is Google’s account of its own systems. It does not say that the engagement rate in your analytics account is that data, so a company should not present an improved engagement rate as a ranking gain. Its value is more direct: it is evidence of whether visitors found what the search promised them.
How the readings change the work
When a page brings visitors from search but few of those visits are engaged, the company compares the page with the queries it is found for, using Search Console. Possible causes include a page that answers a different question from the one searchers asked, an answer buried below the first screen, a slow or awkward page, or a title that promises more than the page delivers. Each has its own fix, and the engagement figures for the same page after the change show whether it helped.
Ask the company which engagement measures it reports, for which pages, whether it has added scroll depth events beyond the default, and how it decides what counts as good engagement for each type of page.