What would happen (theoretically) if someone like Warren Buffett decided to buy $300 Billion of VOO/SPY/IVV?

Theoretical post! (I hope those are allowed)

Everyone knows that Buffett has a large cash position, over $300 Billion.

What would happen if he decided to buy, at once, a large position in one of the major S&P 500 ETFs (VOO/SPY/IVV)?

Is that legally allowed? Would it even trigger? Would the market sky-rocket up?