Each (?=.*X) is a lookahead that requires X to appear somewhere in the string without consuming it — that's how this pattern checks for "at least one uppercase AND at least one lowercase AND at least one digit AND at least one special character" all at once, then the final character class + length bound enforces 8–64 characters using only those allowed classes.
Worth knowing: this validates password format requirements, not actual strength — a compliant-but-common password like Passw0rd! passes every check here while being weak in practice. Format rules and real strength (entropy, dictionary/breach checks) are different problems; a regex only solves the first one.