There has to be another party on the other side of every trade. If someone wants to sell Meta stock for $300, there needs to be someone who wants to buy it for $300.

Exchanges have a collection of their participant's bids ("I will buy for this price") and offers ("I will sell for this price"). When a bid is matched with an offer, the transaction goes through.

Now this is the important part - there are only so many people willing to buy for $300. If there is a mass sell off, all those offers are consumed, and the exchange starts filling the offers of people who would buy for $299.99 (or whatever the next lowest price is). Once those offers are consumed, further down the price ladder we go.

That alone will ensure that not all sellers are getting top dollar, however a market selloff like that would trigger a cascade of lower prices (buyers withdrawing their bids and relisting at a lower price, or not relisting at all), along with things like stop losses and algorithms getting triggered.

There would not be enough buyers. If there were you would have an absurd situation where there is 650bn chasing the shares at that price based on the same public information that is making the “Other half” want to sell everything at that price. That might be possible with more “rigged” markets like cryptocurrency shitcoins. But I can’t see it is possible here.

Picture 10 elephants trying to get through a 4-foot doorway, all at once.

If the volume of sell orders gets too high, the exchange shuts down trading in that stock.

If it's high but not "too high" then the price they all get will be significantly less than what they expected.