How do marketers create demand/advertise something they want to sell?
Once marketers know what they’re selling, the next step they take is identifying the demographic they need to target. Do they need to attract younger people, older people, etc. Based on that, they decide what method of marketing they are going to use to promote the product; for example, if what they are trying to sell caters more towards teenagers, social media marketing would work best.
In saying this, social media marketing has become the best way to promote products for essentially all demographics. Especially now due to Covid-19, people are mainly getting their information from the internet, or doing all their shopping online. 
Have you noticed that in most stores sugary cereal, candy, and such are put at eye level for children, or how basic items like milk and bread are at the back of the store? The reason for this is because it’s how marketers influence us. If a child sees candy right infront of them, naturally they are going to want some and might beg their parents. There’s a science behind this, marketers working for big corporations find a way to create a demand for something that is not always needed. Such as Nestle, Pepsi, Johnson & Johnson, etc .
These corporation usually collaborate with social media companies for profit. Using these platforms, they monitor our activity on the internet; for example, what we search up, what posts we interact with, what we buy, etc. Then, all of this information is collected and fed into an algorithm which then displays what they think you want or will buy. Amazons recommended for you is a good example of this. All of this is because we live in a Capitalist society, where we’re seen as numbers for profit.