Объединить два массива в один по ключам

Возьмите ваши исходные данные

$orders = [
        ['orders' => 5, 'manager_customer_id' => 85],
        ['orders' => 1, 'manager_customer_id' => 124],
        ['orders' => 6, 'manager_customer_id' => 86]
     ];
$data = [
    [
        'name_manager_customer' => "Nick Name",
        'manager_customer_id' => 85,
        'manager_id' => 699,
    ],
    [
        'name_manager_customer' => "Queen Time",
        'manager_customer_id' => 124,
        'manager_id' => 699,
    ],
    [
        'name_manager_customer' => "БигБен",
        'manager_customer_id' => 86,
        'manager_id' => 699,
    ]
];

и преобразуйте первый массив к удобному виду, где идентификаторы будут ключами

$odata =  array_column($orders, 'orders', 'manager_customer_id');

пройдитесь по второму массиву, и расставьте значения

foreach($data as &$d){
    $d['orders'] =  $odata[$d['manager_customer_id']];
}

Если второй массив трогать не надо, то сложите результат в третий

$result = [];
foreach($data as $d){
    $d['orders'] = $odata[$d['manager_customer_id']];
    $result[] = $d;
}

Ещё вариант:

<?php
$a_1 = array(
    array(
        'orders' => 5,
        'manager_customer_id' => 85
    ),
    array(
        'orders' => 1,
        'manager_customer_id' => 124
    ),
    array(
        'orders' => 6,
        'manager_customer_id' => 86
    )
);
$a_2 = array(
    array(
        'name_manager_customer' => 'Nick Name',
        'manager_customer_id' => 85,
        'manager_id' => 699,
        'city_manager_customer' => '',
        'shop_manager_customer' => '',
        'phone_manager_customer' => '',
        'vk_id_manager_customer' => 'just_nick_name',
        'card_number' => '',
        'card_name' => ''
    ),
    array(
        'name_manager_customer' => 'Queen Time',
        'manager_customer_id' => 124,
        'manager_id' => 699,
        'city_manager_customer' => '',
        'shop_manager_customer' => '',
        'phone_manager_customer' => '',
        'vk_id_manager_customer' => '',
        'card_number' => '',
        'card_name' => ''
    ),
    array(
        'name_manager_customer' => '',
        'manager_customer_id' => 86,
        'manager_id' => 699,
        'city_manager_customer' => '',
        'shop_manager_customer' => '',
        'phone_manager_customer' => '',
        'vk_id_manager_customer' => '',
        'card_number' => '',
        'card_name' => ''
    )
);

$result = [];
for ($i = 0, $all = count($a_1); $i < $all; $i++) {
    $result[] = array_merge($a_2[$i], $a_1[$i]);
}
echo '<pre>'; print_r($result); echo '</pre>';