For the record, yes I googled this. I got weird answers and some were talking about movies, is it the same with books too?

Weird answers as in “A saga is a Scandinavian legend” But I have heard someone once say “The Twilight Saga” That’s not a Scandinavian legend is it?

OK, the basics of what you listed:

A series is an indeterminate number of books that all take place in the same universe, sometimes sequentially.

A trilogy is a specific series, with three books.

A saga is basically a very long story with one underlying theme. For example, genera rations of one family, or something like the events of a town or country over a long period. James Michener used to write these. Sometimes a saga can be a series.

An anthology is a collection of work (short stories, poems, and the like) with one theme. Perhaps an author, or a genre.

There can be a lot of overlap.

A “saga” was a long story “of heroic achievement” in a from that became coomon in the Norse countries . Later authors chose to use a simlar stroy structure for other locations and peoples, thus, “Hawaii” was a saga of the Hawaiian islands, Exodus (by Leon Uris) was a saga of the founding of Israel.

Anthologies are often used as textbooks, i.e., “The Norton Antholgy of English Poetry” or “40 Short Stories A Portable Anthology”.

In library science, classification systems put books into categories. The two most commonly used are the Dewey Decimal Classification, Dewey for short, and the Library of Congress Classification, aka LCC. You can read more here. https://en.wikipedia.org/wiki/Library_classification

A saga covers many generations over a time period and the effects on the people involved or the subjuct matter.
Example: ROOTS By Alex Haley
Alex Haley’s family line from ancsetor Kunta Kinte’s enslavement to his descendants liberation.
Very good book and movie.
Trailer :
https://www.youtube.com/watch?v=kL5HC48jG8E