transactions – What literal thing is determining the price of Cryptos / stocks?

The “current Bitcoin price” from an exchange like Coinbase is simply the last price at which a buyer and seller agreed to trade Bitcoin at that exchange, also known as the “last price”. Every time a new trade takes place on that exchange, the price is updated.

The current price from sites that aren’t exchanges themselves, like Robinhood or Preev, are generally taken as the averages of the prices of all of the exchanges they connect to. Sometimes these averages are weighted, so that the price from exchanges with higher trade volume is weighted more heavily than the price from a very small exchange.