While a coin is on its bonding curve, a formula sets the price. Buys push it up and sells push it down. There is no order book and no market maker. After graduation the price comes from the DEX pool, where buys and sells move it in the same way.
Momentum can snowball in both directions. Buying after a big rise or selling in a panic are common mistakes. This is not advice: coins can lose all their value, and you can lose everything you put in.