A foreign/soverign wealth fund is just a normal mutual fund (collection of shares from different companies) that happens to be owned by a government rather than individual investors. What Bernie is proposing is that such a fund is formed, but rather than purchasing 50% of the shares in the AI companies, the government forces the companies to issue them 50% of their shares, for free (thats the "tax"). In practice this means the AI companies will create a new set of shares, equal in number to their existing shares, and then give those to the fund, this will dilute their existing investors by 50%. It is a 50% nationalization of the AI companies. Many other countries have partly and fully nationalized companies and industries, but traditionally the USA has been much more in favor of privatization. The rationale being that free market competition is a much better manager of resources than the monopolistic central authority of government.