How can I Update database mysql by using checkbox and input fields in php

I created a system that will update database, I want to update database by the remaining custody will output from the equation which happens inside $calc_of_custody after I assigned number of custody on an user , I have 3 tables (users, custody, assigned_custody) I tried many ideas but it's only update last checkbox without the above check-boxes , if I have (50 shirts, 100 maintenance tools and 10 hammers ) and the user Edwin will get

shirts 10 maintenance tools 30 hammers 13

I'll retrieve user from users table, custody from custody table and insert them into assigned_custody then update the remaining custody into the custody table, how could I do that ?

the below is snippet from my code

first section is the function.php

function quant_custody($id){
global $con;    
$query = "SELECT quant FROM custody_type WHERE id = '$id'";
$result = mysqli_query($con,$query);
$row = mysqli_fetch_assoc($result);
return $row['quant'];
}
function username_custody($id){
global $con;    
$query = "SELECT username FROM users WHERE id = '$id'";
$result = mysqli_query($con,$query);
$row = mysqli_fetch_assoc($result);
return $row['username'];
}
function name_custody($id){
global $con;    
$query = "SELECT name FROM custody_type WHERE id = '$id'";
$result = mysqli_query($con,$query);
$row = mysqli_fetch_assoc($result);
return $row['name'];
}

second section is a main page:

<?php include_once('../include/config.php') ?>
<?php include_once('../include/functions.php') ?>

<?php

if(isset($_POST['submit'])){

    $cust = $_POST['custody'];
    $usr = $_POST['user'];
    $quant_on_user = $_POST['number'];
    $get_quant = quant_custody($cust);
    $get_username = username_custody($usr);
    $get_namecustody = name_custody($cust);
    $calc_of_custody = $get_quant - $quant_on_user;
    if($calc_of_custody < 0 ){
        echo"there's no enough custody";
        exit;
    }else{
        update_custody_type($calc_of_custody,$cust);
    }



    /*echo $cust." ".$usr." ".$number;*/
    $query = "INSERT INTO assigned_custody (user_name, custody_name, custody_q) VALUES ('$get_username','$get_namecustody','$quant_on_user')";
    $result = mysqli_query($con, $query);
    ?>
        <form action="" method="post">

            <!--
                                                THE BELOW SECTION FOR A USERS 
-->
            <?php
$query = "SELECT * FROM users";
$result = mysqli_query($con,$query);
?>
<select name="user">    
    <?php while($row= mysqli_fetch_assoc($result)){ ?> 
        <option value='<?php echo $row['id'];?>' > 
            <?php echo $row['username'];?> 
        </option>
    <?php } ?>
</select>
                <!--
                                                THE BELOW SECTION FOR A CUSTODIES 
-->
                <?php
$query = "SELECT * FROM custody_type";
$result = mysqli_query($con,$query);
 while($row= mysqli_fetch_assoc($result)){
    $i = 1;
    echo "<br>".$row['name']."  "; ?>
                    <input type="checkbox" name="custody" value='<?php echo $row['id'];?>'>
                    <input type="number" value='' name="number"> <br>
                    <?php

                                         } 
?>


                        <input type="submit" name="submit">

        </form>