Villagers do not build anything, they are always idle (unless interacting with players).
If you build a village yourself, villagers will begin to breed, but only after the village meets certain criteria.
From the Minecraft Wiki:
Villagers will mate depending on the number of valid doors. A valid door is any door (within the village radius) where the number of "outside" spaces within 5 blocks (in a straight line) on one side of the door is not the same as the number of "outside" spaces within 5 blocks on the other side of the door. A space is considered to be "outside" if it is hit by the sun during the day, i.e. it is transparent, and has nothing but transparent blocks above it all the way to the sky. (This means that starting a village deep underground, such as in a cavern or buried ravine, is not feasible.) Notice that a door is not a valid door without a roof on one side, or with equal amount of roof on both sides. Sometimes the villager AI (such as mating and housing) doesn't behave as expected until the game is restarted (as in save and quit, then reload the world).
Once housing has been established, villagers will then mate until the number of adult villagers equals 35% of the number of doors. Any remaining children will grow up as usual, resulting in a total number of adult villagers somewhere above one third of the number of nearby wooden doors. The type of villager that the child is doesn't depend on what type the parents are, e.g., two farmer villagers can have a child that is any of the occupations. Unlike other breed-able mobs, the parents and child have no personal interactions other than socializing. It takes exactly 20 minutes for a villager-baby to grow up to an adult.