In more detail
Google introduced AI-generated summaries in search as an experiment called Search Generative Experience, later launched as AI Overviews. It also offers AI Mode, a more conversational search experience. Google's documentation on AI features and your website explains that both may use a "query fan-out" technique, running several related searches to build a response, and that AI Overviews appear only where Google's systems decide they add something beyond classic results.
To be eligible as a supporting link, Google says "a page must be indexed and eligible to be shown in Google Search with a snippet". There are no additional technical requirements. Site owners who want to limit how their content appears can use the usual preview controls, such as nosnippet, max-snippet or noindex.
Traffic from AI Overviews and AI Mode is counted in Search Console's Performance report within the "Web" search type, alongside other Google Search traffic.
Why it matters when hiring an agency
AI Overviews change what a results page looks like, which can affect click-through rates for some queries. A good agency will look at which of your important searches show AI Overviews and adjust expectations and content accordingly.
Ask how an agency reports on this. Because Google folds AI feature traffic into normal Search Console data, any agency claiming to show you precise "AI Overview traffic" from Google should explain where the number comes from. Be sceptical of anyone who promises guaranteed inclusion in AI Overviews; Google states that meeting requirements doesn't guarantee that a page will be crawled, indexed or shown.