The Forest between worlds is the likely cause or start, perhaps it was Druids who first learned to navigate a Realm between realms as the History of the Library notes no religion being aware of it until after mortals had learned of it.
Druids would add glyphs and sigils as warnings and directions to Trees that never change. What is known is that should you find yourself in that forest it tends to follow you and trees of its kind spring up decades after travelers visit realms they tread on.
Perhaps that Forest and the Library are not connected until mortal hands made them so, perhaps it was trees from the Forest used to make scrolls, books, or even the lumber in constructing libraries and their furniture or art that connected them. Perhaps it is the weight of magic in words pressed together that pinch the pattern of reality close enough that the layers act as one allowing for a straight path to pass through curved dimensions. Perhaps many things added up to reality.
Why the Library was founded is nearly as much Myth as the origins of the Primordials, could have been to amass knowledge and power for its own sake, or to preserve against the collapse of any one civilization. Publicly it is said to safeguard against falling backward into ignorance. Any library be it a single shelf with two books or even a well carved sapling has the potential of being joined yet it is fair to say that is rare indeed.
Once connected the establishment can be made permanent with a teleportation circle as understood by the common laws of magic, yes while magic may change some between realms or spheres there is an underlying tone that always rings true.
Access to the Library and Joining as a member are different and there are complex rules of admission for organizations yet regardless any individual may have a Library card independently of the agreement made by their realm of origin.
Great buildings and complexes of knowledge exist now on many realms for such agreements, staffed such to provide advice on the local development of technologies, not to advance them beyond their means yet to do so safely. As an example they will teach on the effects of radiation and what to do to safeguard against and treat heavy metal poisoning yet never the math needed for reactions. Once that has been learned the library will correct conceptual errors.
One such location is the bases of operations for our adventures on a world nearly dead of life a world lost to an ancient litch who’s end came when the library assaulted and claimed it for its own. Now the great Winter Ziggurat is used by collections as a staging and storage area for receiving and logistics, there are even smaller outposts for farming and research.
Winter’ Reach Ziggurat is also understaffed, having lost their secondary and trisherary teams without having replaced them. The team you are part of is known as Winter’s Grasp a team that rotates it’s division on missions, some of the roster include these PC ideas as well as others you may come up with.
Kuth: Elf of mixed heritage, a staunch supporter of individual rights known for following best practices and a keen eye for safety and betrayal. An At Arms combatant of rank within the organization after their slave master fell in an assult and offered a life with the library.
Elyisha: Born to royalty who refused to be married for political reasons to one of their own relatives. Their magic is strong and of an offensive nature known for wielding a sword made from thoughts and emotions that shines as a rainbow, and as all rainbows are what is commonly called a double rainbow can strike more than one target in a line with a single swing and even at far range.
Elm Fain: Youngest member of the group and little more than a sapling shows talents of healing as powerful as any cleric to a healing god can. Inexperienced and too trusting. Yet able to use divine magic in places thought of lost to gods. So far this is the only life they have known having awakened in the Winters Zigorat only seven years ago.
Sam: Halfling raised by parents of a desert society when they joined the Library. Sam is amazed beyond words by bodies of water be it stream or sea, who’s joy and outgoing nature and social skills get others to talk and say far more than they should is also an expert in regards to inventory, albeit not one in organizational skills. Who hopes to retire into a life of travel yet jokes they will still work for the library in that life also.
Bash: A living magine often thought of a WarFordge they live up to that expectation yet also a well rounded person. Able to speak and command most complex machines finds the ones with intelligence to be arrogant and is possibly the most biased agent of the team with a strong preference to living organic beings, though has never shown any ill will towards elementals or magical constructs. Bash was detached to the Library as part of their world’s membership dues with only two years left on their contract.
Utharian Al’moon Tommallhaka Oustigna riser of the springs holder of the closed door: Has no idea what all of those titles should mean nor do they believe hold any weight. Is of Jin heritage and an Elementalist with a focus on Air and by far the happiest sober person you will encounter. Also on contract yet with the Library directly able to act as a special agent whose loyalty seems only to be that of Good regardless of inconvenience.
Zin: Playful Zin is hardworking and always has the energy to play athletic games with a child-like smile, a descendant of people who field some of the best combatants of their realm. Known for the masterful and quick work with a one handed heavy blade is able to use any weapon or tool as a weapon. Wears a double cover on their head of a scarf wrapping and then a wide brimmed head at an angle.
Mann: Wizard of the most classic sense serving time as a prisoner of the Library for repeat transactions unspecified and assumed to be the clone or some other altered form of the original criminal is the one of the few openly human members.
Rook Rose of the far isles: Ranger hired for wilderness and observations officially yet most know them as the guardian angel whose protective reach is that of a strange stringless bow.
CopperNickle: Constructed by a Bronze dragon to guard and servant outlived their creator who sought out the Library as a new home after thieves assaulted the dragon’s lair for it’s horde. Looks much like a suit of half plate with wisps of light between always has the right tool on hand when called for and able to see how to use the environment and what is at hand to Devastating effects from intellect alone.