I mean, I'm not the person these questions are addressed to, but this isn't really an effective way of communicating socialism or questioning capitalism, so to answer some of your specific questions:
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Regulations exist to prevent "one man owning everything in law" Anti-monopoly laws have worked to an extent in the past. They're stripped away or gone unenforced in late stage capitalism, but you specifically asked what law stops it... those laws. I'm not saying they are a sufficient or permanent solution, just answering the question.
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No law exists that limits the borders of nations... but that has nothing to do with capitalism, nor is that solved by socialism. If anything, late-stage capitalism has made a lot of borders relatively stagnant, as international corporations freely cross borders without issue, and abuse them for easily exploited labor and tax havens etc.
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Capitalism isn't strictly speaking a system where "one man owning everything" is a feature... that's just a dictator. This isn't something that any capitalist would think they support, so asking the question like that doesn't really open up the discussion.
Capitalists are more than happy, for example, to run franchises, where they exploit all the revenue but take on none of the risk of ownership. Or to use third party contractors, to take on none of the risk of lawsuits or unions. Or to sell part of their company stock to the public, to ensure their wealth and revenue stream is propped up by the wealth of others.
In short, capitalism isn't about owning everything... it's about extracting as much of the available wealth as possible with as little risk as possible. Which isn't limited to capitalism, but it is heavily incentivized by capitalism.