When you have a product and people buy the product, they are your customers.
When the buyer tells you what they want and you make it for them, you have a client.
The big difference is who makes the decision regarding what you create. If you create it and the buyer only has the choice of to buy or not to buy, you are making the decision on the creation and they are a customer. If the buyer is the one who dictates the product and you only have the decision on whether to make it or not, then the buyer is making the main decision and they are your client.
It might not seem like a big difference but it’s important to know whether you have customers or clients.
Have a great day!