These days there's online discussion about use of artificial intelligence to write news articles. I like to think of a SERP as a topic-specific webpage - like a sort of article on a particular topic to be found on PC Magazine or CNET News website - except that it has been rapidly assembled from scratch, and it has been built by machines.
The screenshot below of Bing makes this thought somewhat clear. To a user, Bing instantly wrote a dedicated webpage for the query windows defender for windows 8. This custom-built webpage contained a lot of useful information related to the query. To be sure, this SERP certainly wasn't as lucid or thorough as a human-built webpage would be for this same query/topic, but that quality gap is only a matter of time. The core principle prevails and is time-independent - that SERPs are fundamentally custom-written articles/webpages in response to queries entered by users of Web search engines.