Change default login auth

You can hook into this filter – https://developer.wordpress.org/reference/hooks/check_password/ – and validate if the user submitted password is the same as the user’s custom code field value return true.

Example:

add_filter( 'check_password', function( $check, $password, $hash, $user_id ) {

    // Run additional checks if current check is false
    if ( ! $check ) {

        // Check that password matches custom field and if so return true
        if ( $password && $password = get_user_meta( $user_id, 'CUSTOM_CODE_FIELD_ID', true ) ) {
            $check = true;
        }

    }

    // Return check value
    return $check;

}, PHP_INT_MAX, 4 );

Now…this is just a simple example of the code you may use. But you would likely want to secure the “code” better using some sort of encryption. WordPress stores passwords as md5 hash values but it’s not as secure as other methods such as bcrypt.