Bernie is not proposing spending any money on acquiring these shares.

He is proposing simply seizing half of them.

So, the government gets a half stake, a controlling interest in these companies, for free, because we are the government and we say so.

So, that would mean the government can then directly regulate these companies, and if they become profitable, those profits can be poured into a soverign wealth fund, which could fund ubi or universal healthcare, etc.