If Karl Marx said socialism is when the state does stuff, the Adam Smith said...

Going to disagree with Smith on this one. It either becomes a monopoly or an oligarchy.

I hear a lot of libertarians argue that government is the only thing that creates monopolies but in reality the private sector can also allow monopolies to thrive just from private businesses growing in the free market they are intended and do deals with smaller businesses that are unable to compete with them and act upon their voluntary economic freedom to buy out smaller competitors. Funny how libertarians advocate for unregulated free market capitalism to allow businesses to grow as large as possible yet they act surprised when that happens and it has negative consequences and even try to make it like a was really government that caused the problem. Sometimes the even the free market can destroy the very businesses that thrive in it. During the Theodore Roosevelt administration, giant monopolies were killing the free market and there needed to be government intervention to break up the monopolies and restore the free market. I would debate a lot of libertarian/ancap types on the flaws of completely unfettered capitalist and they would accuse me of being an anti-capitalist socialist even though I don't adhere to extremist ideologies like Marxism or even "democratic socialism", they didn't like to admit that their libertarian ideas are not always right and even resorted to acting like an asshole to me calling me a "communist" "socialist theif" "anti-freedom" "economically illiterate" and "retarded".

And prior to Roosevelt two of those monopolists were practically oligarchs because of how they influenced government policy especially on the federal level.

What exactly did they do? Did they lobby for corporate tax cuts?

Bought and paid for candidates on a scale that may or may not be worse than today. I forgot which monopolist but one of them literally bailed out the government's debt.