"Parts" is a symbolic way to describe different aspects of the personality. Of course, these are not physical parts that assemble our minds the same way a machine parts assemble a machine.

Describing our mental and emotional processes as "parts" has nothing to do with "blaming" anything on anything and relieving the person from responsibility for how they behave. It's supposed to help one understand their own behavior in terms of its underlying motives. Regardless of the motives, one's choices belong to them, so they are always responsible for how they behave.