How exactly is breast cancer transmitted?

Multiple reasons: Most likely multiple factors at play including genetics, exposures (including alcohol), and hormonal factors.

Not transmitted: It is essential that you know that breast cancer is not "transmitted" like an infectious disease. It is not contagious (though there is a mouse form that is passed mother-to-daughter). A woman develops breast cancer through mutations over the course of many years. Hope this is the answer you sought, and good luck.