It can host large datasets as well, yes. It is hosted by CERN, so it is not specifically IT in any way. It also allows you to restrict access to the files of your submission. It has no requirements to submit your LaTeX sources, any PDF will be fine. There are also no restrictions on who can publish. You'll get a DOI, of course.
Everything published on arXiv could also be published on Zenodo, but not the other way around.
It is just a preprint repository. It is pretty open (the stories where a preprint was rejected or delayed unreasonably are extremely rare). It offers the basic services for a math/compsci/physics themed preprint repository.
I don't see much of a monopoly, nor any "moat" apart from it being recognised. You can already post preprints on a personal website or on github, and there are "alternatives" such as researchgate that can also host preprints, or zenodo. There are also some lesser known alternatives even. I do not see anything special in hosting preprints online apart from the convenience of being able to have a centralised place to place them and search for them (which you call "monopoly"). If anything, the recognisability and centrality of arxiv helped a lot the old, darker days to establish open access to papers. There was a time when many journals would not let you publish a preprint, or have all kinds of weird rules when you can and when you can't. Probably still to some degree.