>But you'll argue that corporations are nothing but the consequence of a corrupted non-voluntary state
Well in the case of the USA, whenever the workers did try to organize (whether unions or socialist crap) it tended to be the state or city governments that hired private mercenary groups/protected them under law, so yeah, it's kinda hard to not say it was the result of an involuntary state.
>how do you demonstrate that the fusion of many privates (forming a big monopolist corporation) couldn't take place in ancapism?
I'm not saying it couldn't happen, I'm just saying it's unlikely. Just as bottom-up structures tend to start collapsing as they get too large, so too do top-down structures once they reach a certain zenith. We call this diseconomies of scale, in which the growth strains of a business actually begin to hurt production, worker morale, innovation, etc. more than the expanding profits help.
Also, a monopoly/oligopoly is not inherently a bad thing in a market setting. Assuming the monopoly is backed only by its shareholders and the money it can produce rather than government privileges, it has to continuously innovate or otherwise provide either a superior good/service or cheaper good/service to their consumer-base in order to stay afloat. It will never be the best per-say, and other companies will be created to fill those niches/challenge the default monopoly's market share, but it will improve. If a monopoly forms, it will likely look similar to what we refer to as a "star bus topology" in networking. You have a baseline product that consumers want (the main bus), the monopolies (the main network routers/hubs), and then the specific products offered by the monopoly's competition (the PCs and/or sub-hubs).
>how do you demonstrate that every (or at least, the most of) corporations arise from the corrpution of non-voluntary ruling states?
Well to begin with, a corporation is a legal entity that requires a state to exist in the first place. Its literal definition is a company or group of people authorized to act as a single entity (legally a person) and recognized as such in law. You can read up on it more on wikipedia if you want: https://en.wikipedia.org/wiki/United_States_corporate_law
Now if you're asking about large companies rather than corporations specifically, then there's a few examples of monopolies arising without the state (de beers and the early stock market are the most prominent), but these were typically niche fields that were hard to get into/worked better under a monopoly than under other programs, or were created by trade secrets.