Please I need clarification here, how come the "where" clause came after "group by" as it should be before it and if we need to use aggregate function at that place after "group by" so we can use "having" clause, please correct me if I'm wrong

Oh very nice catch! I didn't actually notice that but you're 100% right. Where should come before group by and having comes after. I will make a note of this in the lecture.