IWC is designed to be easy to learn. It includes elements of the languages of all species known at the time it is first devised. More worlds are subsequently discovered/contacted/involved, meaning IWC is almost immediately out of date.
IWC is a partial success but has some drawbacks: it is hastily put together, on the principle that some form of language would be better than none. It is also rather obsessed with featuring all the (original) source languages equally, making it more complex than necessary, and so it isn't as easy to learn as it could have been.
The society [world/country as yet undecided] responsible for Interworld Common recruited linguists from all the planets involved in the worldgate events at that time. (Shade's contribution was a group of goblins from a university somewhere in the green-majority western area of Terrimoire.)
IWC was a rush job. The basics were set down over a few years and released, leaving the users of the language to add most of the vocabulary from loan words, popular slang, compound words...
IWC was intended to be apolitical, to avoid favouring one culture's language over another. Of course this was only partially successful: IWC was tied up with the politics of the liberal geeks who designed it, leaving it open to criticism on that score.
The creators came up with a small number of manuals, crate-sized collections of binders. These were intended to be shared around the involved worlds and kept updated and synchronised by dedicated teams [to what degree of success is unknown!]. Imagine the security and attitudes around these as comparable to something like Earth's [standard kilograms] - precious things but intended to be used to help.
Normal learners of the language would not use the manuals, but instead whatever teaching aids or reference materials had been made. [Expect this to vary from place to place. Politics, mass printing techniques etc.]
First Interworld Common is gradually replaced, by the dictates of fashion as much as any serious consideration of its merits, by TWIC some time after the finish of the War.