Most codes are written for a first order: they give a new shopper a percentage discount, and the discount is capped at a maximum amount. Even when several codes sit on this page there is only one first order, so in practice one of them applies. The discount does not cover some product categories, so not everything in the basket may get cheaper.
The code is entered at checkout. Register in the app or on the website, add products to the basket, then type the code into its field and check that the total has dropped. Orders are collected from a pickup point.
The official Wildberries code ends on whichever comes first: the pool of codes running out or the announced end date. A code can therefore stop before that date, so look at both the date on the offer page and the total shown at checkout. The marketplace itself can change the terms. Offers from other shops sit in the fashion section.