Gary Illyes from Google took a shot at explaining how he sees Google's core updates. He said on Twitter "when i say "core algorithm" publicly, i mean the collective of algorithms that are part of the scorer (IR score in information retrieval) & generally unnamed externally."
IR scorer is Google's Information Retrieval scorer, basically the fundamentals of how a search engine ranks documents in search.
Previously, Gary has been quoted as saying these are baby algorithms, a bunch of smaller algorithms that act in part of a bigger goal.
Here Gary is saying the core algorithm is a "collection of algorithms" used by Google to rank sites. These algorithms are generally not named by Google and thus no need to be specific with the public about which algorithms specifically were updated when a core algorithm update is released.
Here is Gary's tweet about this, with his disclaimer:
Again, this is just my take and others might have a different take.
— Gary 鯨理/경리 Illyes (@methode) May 15, 2020
Forum discussion at Twitter.