psychiatrist question: on my way to becoming a mental doctor; quick question?

in order to become a psychiatrist which is a mental doctor, you must have a BA (chemistry/psychology etc) I researched and apparently you are not required to learn a specific science in University in order to get into medical school? im just a little confused about that because medical schools require science related courses so...how does that make sense?

also, you are obligated to goto medical school inoder to become a psychiatrist. so my question is..if its a mental doctor does the curriculm in medical school, differ from other students that plan to become a family doctor? since a mental doctor only deals with mental problems.. OR is it the same, you just..specialize in mental disorders etc later on..which basically means that u must study even longer since its a specialization? and I live in Canada, Ontario