Do economists use microeconomic theory in real life?

Microeconomics is concerned with the action or behavior of an individual or business. It is not concerned with the action of the whole economy.

Yes, economists use micro economic theory in real life. Microeconomics shows the demand and supply of any good in the market, and economists are concerned about the whole economy, so economists have taken microeconomics as a primary source while making any decision about an economy. One of the uses of microeconomics can be seen while talking about scarcity. Scarcity is a micro economic term which is highly used by economists in their theories. Scarcity is a term which represents limited resources in an economy. Every individual has unlimited wants and scarcity occurs when people demand are difficult to get fulfilled.

So while making any economic theory, an economist has to take a look upon microeconomic factors too with the macroeconomic factors.