How react-redux works?

Think of a small store, you are the manager of that store(you maintain a ledger for the inventory and sales), a guy comes in, buys a chocolate, what 'actions' you would take, decrease one chocolate from inventory, add the price of the chocolate to your income. Now, you are given the task of developing store's website, you do that obviously with react and redux, redux maintain the state of your application, any change is done via 'dispatching an action', so you would have an initial state say, {chocolates: 25, income: 0 } as the customer buys a chocolate, you would 'dispatch two actions, decrease quantity & increaseIncome. Now every dispatched action goes to a reducer where state is maintained, and once a reducer is called it takes care of all the places where your state is being used, say in display of chocolate page you would say, 25 chocolates are remaining, once a customer buys a chocolate and you dispatch an action, or decrease quantity, it would automatically be changed to, 24 chocolates remaining.

What's the use of an article tag?

Article tag is used to specify that the content is self-contained, what that means is, it doesn't depend on the rest of the website, if it's taken out of the website, it would still make sense. Some examples of articles could be, blog posts, news story, forum posts etc.

Explain CSS box-sizing property.

By default, when you set a width of an element, it's actual width is calculated as, width specified + padding + border, but if you want to restrict padding and border in the specified width, you can set the box-sizing property as border-box.