Get the PK from one table and Insert As FK into another

I have an inheritance in my work and I'm having trouble making it work. Tables Example

CREATE TABLE IF NOT EXISTS pessoa (
  idpessoa INT NOT NULL AUTO_INCREMENT,
  tipopessoa VARCHAR(45) NOT NULL,
  PRIMARY KEY (idpessoa)
);

CREATE TABLE IF NOT EXISTS juridica (
  idjuridica INT NOT NULL AUTO_INCREMENT,
  cnpj VARCHAR(25) NOT NULL,
  inscrestad VARCHAR(45) NOT NULL,
  razaosocial VARCHAR(150) NOT NULL,
  nomefantasia VARCHAR(150) NOT NULL,
  pessoa_idpessoa INT NOT NULL ,
  PRIMARY KEY (idjuridica),
  FOREIGN KEY (pessoa_idpessoa) REFERENCES pessoa (idpessoa)
);

Well, this is a part of the inheritance that I have, I'm trying to register a Legal person, but first I have to register the Person. I'm using PHP (learning now) and the code is the following for inserting a legal person.

Connection to the bank:

function abrirBanco(){
    $connection = new mysqli("localhost","root","","apsbd1");
    if($connection->connect_error){
        die("Conexão com o banco falhou: " . $connection->connect_error);
    }
    return $connection;
}

Person and Legal INSERT:

function inserirPJuridica(){
    $banco = abrirBanco();

    //Insere uma Pessoa;
    $sql = "INSERT INTO pessoa (tipopessoa) VALUES ('Juridica')";
    $banco->query($sql);

    //Seleciona o ID da Pessoa;
    $sql = "SELECT idpessoa FROM pessoa ORDER BY idpessoa DESC LIMIT 1";
    $res_idpessoa = $banco->query($sql);

    //Insere a Conexão entre Pessoa e Jurídica;
    $sql = "INSERT INTO juridica (cnpj, inscrestad, razaosocial, nomefantasia, pessoa_idpessoa) VALUES ('{$_POST["cnpj"]}', '{$_POST["inscrestad"]}', '{$_POST["razaosocial"]}', '{$_POST["nomefantasia"]}', '$res_idpessoa'";
    $banco->query($sql);

The person I can insert into the bank, but when I insert the Legal it gives an error

Catchable fatal error: Object of class mysqli_result could not be converted to string in C:\xampp\htdocs\apsbd1\inc\funcoes.php on line 302

Line 302 is the line of $ res_idpessoa = $ banco-> query ($ sql);

Probably the query is not a string that can be used in an INSERT, how do I then insert the person id from the person table into the legal person's person?