Could someone explain to me where and how do dropshippers pick products? Im so confused...

Hey yall,
So, ive kinda been looking lately into print on demand and obviously i also stumbled on dropshipping.
If i correctly understand, the main difference is that with print on demand i have the power do chose my designs and with dropshipping i do not but then how do you even sell with dropshipping?
Is dropshipping just kind of re-selling items? For example if there is a cofee mug on amazon for 10$, you connect with amazon, put the exact same product on your e commerce store and sell it for, say 15$?
And when you get a buy order you receive the 15$ of which you get 5$ and the rest goes to the original seller with the original price?
Im basically confused as to where and how do you then pick your products if you have no control over your designs and the physical product already exists.

Thank you for your responses.