Basically if it's the first and earliest record and the one that everyone looks to as the most accurate source it's canonical.
The latest commit where the master branch finds itself in isn't any more "true" or genuine than the first commit in history, nor the commits on a fork, nor the commit on branch-1, nor the commit on branch-2
If it's all fiction anyway there is no truth out there for the map to be compared against. All permutations are real, all maps are the territory
I do find it amusing, however, that depending on context people consider the first commit in history to be canon, while on other cases they prefer the newest commit approved by the Authority™
I know some people hate those franchises but like it or not, they are some of the biggest mythologies that came out in our time.
Until even the original author retcons the heck out of their world and goes against established, explicit, well explained lore
Star Wars and the MCU are awful examples to use for the idea that canon can be used for consistency and authoritativeness
For the MCU, yes, by law Disney has authority over the MCU.
Most notably, for the actual list of books there's the Catholic Church for the Catholic canon; the Masoretes for Judaism; Synods for the Greek Orthodox canon; and a broad consensus in the Protestant community.
For the texts themselves, the Septuagint and the Masoretic text are the only game in town for the Hebrew Bible, while the Greek texts of the New Testament were very firmly set in stone by the Imperial Roman Church (as central of an authority as it gets).
Note that some of these authorities (the Masoretes, the Imperial Roman Church) do not exist anymore. THat just means that canon is unchangeable, it still exists.
No one holds authority over the original texts. There is authority over translations though. But the original is open source with no license.
You can make an edited version of the original Hebrew and Greek scriptures with whatever changes you want. But no one will agree to call it The Bible, because it does not match the text endorsed by the original authorities.
Same as how there still exists a Sherlock Holmes canon based on Doyle's work, even though you can legally write whatever you want even in contradiction to that canon.