I'll give you an example of using the MERN stack. You create a backend and create an order model, routes, and controller(where you make CRUD operations- check CRUD if you don't know yet).
Using Express.js, you create an API to talk between your frontend, admin panel, and backend. Creating an order is creating a request to the backend from API using the frontend. But, to better understand it, I suggest you try creating with tutorials and start with understanding CRUD.
Think about a restaurant. Frontend is where people eat, the backend is the kitchen, the database is the larder or the depot of it, and waiters that creates communication symbolize API, or HTTP requests.