When a father divides the house, is the apartment in which he married his son included in the division, or is it considered part of the father's worldly expenses for him?

The default position is that the apartment remains the property of the father simply by his permission for the son to reside in it. This is not considered a gift. Even if he ordered him to build on land he owns and permitted him to reside there, it is not a gift. A gift does not become effective except with the consent of the other siblings, because the preponderant opinion is that it is obligatory to equalize among children in giving, and preference is not permissible except for a legitimate Shar'i reason. Dividing wealth during one's lifetime as if it were an inheritance is not valid.