These accounts are heavily colored by the expectations and the worldview of those monks, and we do not know where the ideas that the Norse actually believed in end and where the ideas of what the monks thought religion is supposed to be begin. For example, much of the popular conception of Ragnarok is heavily influenced by Christian eschatology, instead of the original Norse beliefs. To complicate it even further, the Norse beliefs were in no way static, and as the influence of Christianity spread, the beliefs might have morphed to absorb concepts from Christianity.
In more easy words: christians believe that one day the world really ends with judgment day/apocalypse, and interpreted Ragnarok similar as the end of the world, which is what many people today think of Ragnarok (and that view that gets reinforced by the popular movies)
But Ragnarok is not the end of the world, just the end of a cycle and start of a new beginning. The snake that bites its own tail. Endless cycle of seasons. Really a different philosophy.
https://www.youtube.com/watch?v=zbT8vzX4sZY
(Einar Selvik from Band Wardruna explains it, before performing a song about it)
There is also the interpretation that we live in a post-Ragnarok world, which conveniently allows the Christian narrative to perfectly mate to the end of the Norse gods’ reign.
There are even 6th century writings referencing a germanic mythology with many similarities to the norse mythology.
First of all, it's a branch of Indo-European mythology, as are the Greek, Roman, and Hindu pantheons. So it has existed in some form for thousands of years. But "Norse mythology" as we know it was mostly written down in the 13th century - so 700+ years ago.