The data in B+Tree stores the last_name, first_name, and dob field information, which is similar to a field in a primary key index. The three fields are combined together. The difference between the comparison and a single field is that the first comparison is Last_name, then compare first_name, and finally compare dob, and execute this rule.
To sum up: This is a better understanding of the principle of the leftmost prefix of Mysql index