Supply and demand are defined as the willingness and ability to transact at each price. People who want something but who aren't willing or able to buy it at a given price are the cause of the supply curve. The marketplace is full, at any time, of people who would buy or sell if the price was right. That's why they're curves and not points.

Here is a free textbook that requires no more than highschool math: https://openstax.org/books/principles-economics-3e/pages/3-i...