How to make really realistic houses in Unity, or do they use other programs for it? Do they use cinema4D or 3ds max or what?
You need to use modeling program or get model from other people (free sites/ purchase).
You can use blender, its good and free.
You cannot create realistic houses model in Unity.As you shd be knowing that unity is a game engine.You can create basic structures but if you want realistic models then you shd use design softwares like Maya , Studio Max ,Blender[free software] etc.
After creating the model and texture , Unity provides you beast lightmapping to give you baked beautiful light mapping for the model. Or you can also import already baked textures[Using V ray or mental ray] into unity.