Tell me about the Celestials

So, like many, my first introduction to the term "Celestial" was Knowhere in Guardians of the Galaxy, which is said to be the head of a Celestial. Then I'm pretty sure they state that Ego is one in Guardians of the Galaxy Vol. 2. Since Ego was one of a kind, I took it to mean it as something of a blanket term to cover great cosmic beings like The Watcher, etc.

But, in order to prepare for The Eternals movie, I started reading The Eternals comic book, and the Celestials therein are definitely their own species, of which Ego is not a member.

I decided to look up if Knowhere existed in the comics. It does, but it has a weird section on the wiki page on how it's first appearance was "historically" in 2008, and "chronologically" in 2018, well after it was intoduced in the movie. So what does this mean? Was it invented in the movie and retroactively stated to have been in the Marvel Comics Universe?

At any rate, it seems that Knowhere was always meant to be the head of an Eternals-kind-of-Celestial, and Ego was not.

Chronologically means... chronologically, the earliest appearance in the timeline and not in terms of publication. So in this case a flashback to the scene of the head being cut off.

And yes, movies take many liberties with the comics (and there are retcons in comics too), comics Ego is not a Celestial. The Eternals wasn't originally intended to be a part of the Marvel Universe, it was only Roy Thomas in Thor (with Grunewald and Macchio finishing the storyline) that made them fit in the Marvel universe. This is Thor Annual #7 for the set-up and #281-300 for the storyline. It's honestly not a good story, since aside from merging Eternals into MU, Thomas uses the arc to straighten a ton of Thor's continuity weirdness so it's a ton of exposition.

A significant amount of what defines Celestials today in comics comes from what is actually an AU and not the main universe. In Earth X (and its sequels) it's shown how Celestials are tied to basically everything at Marvel. A lot of elements from this mini were then brought into the main universe by other writers, especially Hickman and Remender. In effect, while Eternals were largely irrelevant, Celestials introduced in their series that wasn't originally intended to be a part of Marvel canon became central to it thanks to the AU series.

The modern origin of Celestials was introduced in the Ewing's Ultimates/Ultimates^2 that started in 2015. It provides a broader explanation of their importance in the cosmic hierarchy, and since it's tied to the Multiversal history itself it should technically be also relevant to MCU Celestials (but it probably won't).