It’s not dissimilar to a commodity like Gold with some extra parameters like number of stocks. Comes down to an open market and buy/sell demand. There are other parts to your example such as depth of order book (orders on the buy sell side) which provide liquidity. So if a low volume stock is at $4 and you offer to buy at $100 you would probably end up with 100/4=25 shares. If was more illiquid you could possibly move the price up depending on order quantity.