one person writes a story and that story exists within a world that is gestured to by the outer rims of the narrative. there are events occurring in the margins of a fictional story that inform the shape and feel of the external world. but the 'truth' (so to speak) of what is occurring 'offscreen' is only known by the author, and sometimes not even then.
when a sequel or a prequel is written from the perspective of a single author that builds on that same single author's previous work, in effect the reader is experiencing another window into the same world.
in my opinion, a good sequel/prequel builds on what the previous work/s established and does not contradict them, but adds something to the fictional world. most of the time that something is additional linear narrative but sometimes it's not - sometimes it's a cookbook or a history book or a set of illustrations that add more detail to the fictional world.
but ultimately the effect of these disparate pieces of loosely connected creative output is that they are woven together to create a larger and more coherent picture of a shared world between the works. the truth of the world, insofar as that exists, is still being constructed within (and thus for all intents and purposes is governed by) a single author's mind.
the modern convention of 'shared canon' in a story-developmental sense is a useful construction that performs, in effect, a magic trick - it makes the work of ten, or fifty, or three hundred individual creatives feel like the work of one. it makes six creative works by six different creative teams all contribute to a greater overarching narrative, or at least to the truths of the shared world that all these creative works "exist" "within".
this is the magic trick that undergirds who knows how many modern "shared universe" media IPs. something may not be by the original creator of the original work, but it is made to feel like it's part of the same series.
it's not an easy magic trick to maintain, and less-effective executions are easily spotted. successful execution requires a massive amount of coordination, usually some combination of centralized recordkeeping, strong understanding of individual roles in the creative process, and extremely tight communication between creative teams - even moreso if they're using different forms of media that require different development schedules.
the scale of work required to maintain the magic trick rises exponentially the more ambitious the quantity of work expected. the scale of work required to maintain the magic trick rises exponentially the more ambitious the quality of work expected. it's just not something you can take lightly or assume requires minimal care.
when it works, it feels natural. when it fails, it fails with the same lack of verisimilitude of watching a magician reach for a ribbon that isn't there. a failure leaves you wondering if the performer forgot the ribbon, or worse: simply didn't care to finish the trick.