Venus is thought to be subject to a runaway Greenhouse effect, first proposed by Carl Sagan in 1962. The Greenhouse effect is why a car can become quite warm inside if the windows are shut up in spite of the outside air being chilly. For a car sunlight enters through the windows and warms the interior. Instead it is trapped which makes the car heat up. For Venus it is the CO gases that act like a car window.
Some visible light penetrates the clouds and heats up the surface ground.
The ground emits infrared but cannot get out. The surface heats up. The surface heats up enough so that the amount of energy in visible sunlight that is absorbed by Venus equals the amount of infrared light leaking out from the planet. This is called radiative equilibrium. If Venus and Earth are so much alike in many ways, why are their atmospheres so different.