Grocery delivery service - how to create?

Question

Startups

Good evening, there was an idea to open home delivery of products from supermarkets, but if everything is more or less clear with the organization of delivery, it is not clear how other grocery delivery services receive information about prices and availability of products in the store? For example, if a client wants delivery to be made from Lenta supermarket, he goes to the catalog on the delivery site and sees all the goods from the Lenta store. How delivery services get this information. Sorry for the stupid question, I just don't understand

Answer 1

And you try to place an order and see how it works.
I myself often order from Lenta. Making an order. Then the order picker calls me and says that he will now start collecting my order and I need to be ready to answer his calls. If there is no position from the order, then he calls and informs about it, offers alternative positions from those that are available.
It should be understood that it is very difficult to get the actual balances at the current time in large hypers. Yes, and it makes no sense.

Answer 2

Contract -> API

Answer 3

They never receive. The courier sends you a WhatsApp message 10 minutes before he enters the supermarket, saying that I will start buying your products, please be in touch. If there is no item from your order, he takes a photo of the shelf with the goods and then you choose from what is there.