I tried to coding it. I am still getting stuck over it. The main goal was if user select value from mysqli database selected it and send the values to other pages. I know people recommend it use by AJAX. I tried to use it. still not working. I'll put details code below.
Main pages Code(main.php)-
<?php
    session_start();
    $conn=mysqli_connect('localhost','root','','user');
    if(!$conn){
        die('Please check an Connection.'.mysqli_error());
    }
$resultset=$conn->query("SELECT name from newtable"); ?>
<!DOCTYPE html>
<head><script src="https://ajax.googleapis.com/ajax/libs/jquery/3.5.1/jquery.min.js"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.16.0/umd/popper.min.js"></script>
</head>
<body>
        <center>
            Select DataBase to Insert it<select name="tables" id="tables">
                <?php
                    while($rows=$resultset->fetch_assoc()){
                        echo'<option value='.$rows['name'].'>'.$rows['name'].'</option>';                            
                    }
                ?>
            </select>
            <a href="database1.php">click </a>
        </center>
    <script type="text/javascript">
    $(document).ready(function(){
    var search='';
    $("#tables option:selected").each(function() {
        if ($(this).attr('value') !== '') {
            search=$(this).attr('value');
        }
    });
    $("a").click(function() {
        $.ajax({
            method: 'post',
            url: 'database1.php',
            data: {key:search},
            beforeSend: function() {
                $('body').css("opacity", "0.3");
            },
            success: function(response) {
                 alert(response);
            },
            complete: function() {
                $('body').css("opacity", "1");
            }
        });
    });
});
     
    </script>   
</body>
</html>
as alert box i am getting value of it but second pages get error that key value doesn't exist. here the second one pages (database1.php) -
<?php
    $conn=mysqli_connect('localhost','root','','user');
    session_start();
    if(!$conn){
        die('Please check an Connection.'.mysqli_error());
    }
            $database=$_POST['key'];
            echo'You Selected'.$database.'from table';
                $sql = "SELECT * FROM $database";
                $result=mysqli_query($conn,$sql);
                if($result){ 
                    echo'Worked';
                }else{
                    echo'ERROR!';
                }
?>
so what the problem occurred?
UPDATED ANSWER
Thanks to @swati which she mentioned that use form tag instead of AJAX (i know its simple answer) still by the way thanks for answer. :)
UPDATED CODE FULL -
    <body>
    <form action="database1.php" method="GET">
    <center>
            Select DataBase to Insert it<select name="tables" id="tables">
                <?php
                    while($rows=$resultset->fetch_assoc()){
                        echo'<option 
              value='.$rows['name'].'>'.$rows['name'].'</option>';                            
                    }
                ?>
            </select>
         <input type="submit">
        </center>
</form>
</body>
SECOND PAGE(database1.php) CHANGES LITTLE -
$database=$_GET['tables'];
 
    