Core updates are "significant, broad changes to [Google's] search algorithms and systems." [
developers.google.com...]
While a "rearranging" of the SERPs can certainly be a part (or result) of that, most of the changes are more likely to affect how content is assessed. But I think that increasingly those two parts are more difficult to separate, everything being relative to the query.
Organic serps begin here.
Strictly speaking, images, videos and "people also ask" are also part of the organic SERPs.
The core update didn't change the ranking of the page, it changed the layout of the SERP.
Doesn't have to have been a result of any "core update".
Some of my most important positions are below image results; they never used to be until a few years ago, when Google started catching on to what sort of content people were looking to find. So I understand why they're there and it tells me something about what I need to do to keep competing and provide a better experience. I don't believe the image results are set in stone there at the top, I think I can regain the top position once I do a better job of meeting query intent for as many users as possible. Analyzing the search results has helped me plan for the future.
It's important to realize that "rearranging" is not done arbitrarily, for each query Google's trying to learn what sort of content the user is going to be most content with. Some queries are simple in that regard (i.e. clearly people want video, or information, or maps, etc), others more complex (and you'll see a wide variety of results). If you can catch on to what Google is learning, you might have a chance to compete. That's not to say they're perfect at it, of course; but that's the direction they're heading in (as they have been telling us for many years).
You might find, for example, that for "widgets" there are images, videos or what-have-you at the top, whereas for "silly widgets" there are not. Then for "broken widgets" there's a "people also ask" section. See what can you learn from all that, and try to understand why things are "rearranged" when they are.
