How does “reserving” a product during checkout work in online stores so that other users cannot redeem this product?

Question

Electronic commerce

I'm trying to figure out how and when to reduce the available quantity of goods on the site for other users, when any of the users is at the stage of placing an order.

Situation: customer A put the goods in the basket in the amount of 1 piece. (only 1 pc is available in the store), being at the stage of placing an order, it turned out that the goods were no longer available, since while buyer A entered the data, buyer B had already managed not only to enter them, but also to pay for the goods.

Question: at what stage is the product attached to the buyer (to give him the opportunity to pay for what he put in the basket), so that when the exchange with the 1C website is working, there will be no problems (where to change the values ​​\u200b\u200bof the quantity of goods in the warehouse: only on the site or in and 1C)?