What does optimal mean in this scenario?

That’s a great question and it really depends on a lot. Ideally if there’s no artificial monopolies and there is significant competition, all the sellers of a good or service will be forced to offer the lowest price, in order to win over the buyers, who will buy the cheapest product. Of course at some point the price cant get any cheaper, and that optimal price will be slightly above the cost of making the good.

The consumer doesn’t really have power in that system in any way that matters.

The consumer can choose not to buy a product they don’t like, and if they want to amplify that effect they can organize boycotts of it. Imagine if Amazon doubled all of their prices tomorrow, no one would buy from them, and they would lose money. And because of this, the market can somewhat control Amazon’s prices. if there’s an industry that is filled with expensive, low quality products everyone hates, there is massive incentive for a new player to join with a product that is cheaper and better.