Dynamic content template

I need some help with the structure of some dynamic content in my custom template, for my WP site.
I want to show a drop down menu (FORM) and have the user select a value. Then show a different form that has results based on the query of a custom DB using the initial selection. This form allowing some selections and comments, before writing back to the DB.
I have done this before in PHP but not in WordPress. My FORM works perfectly as does the reading and writing to the DB.
My issue is that I can’t get the FORMs to display. I have tried to use a global variable to manage the display of each FORM. The primary will display, but upon selection it will not show the second FROM.

My code below, note I have removed the bulk of the HTML and PHP code to make it easier to read:

    global $wpdb;
    $GLOBALS['Air'] = '';
    $Air = $GLOBALS['Air'];
//<!------------------------Create FORMs--------------------------------------------------->
//Always show the form
echo '<form action="#v_form" method="post" id="v_form">';
If ($Air == "")
    {
    //    Primary input
    //    Create HTML drop down list from custom DB. Select one item from list. (works)
    }
   
If ($Air != "")
    {
    //    Secondary input, check box selection and comments field
    //    Query DB based on previous selection of "Air", show results and allow user input. (works)
    }
    ?>
    <tr><td></td><td><input type="submit" name="submit_form" value="submit" /></td></tr>
    </table></form>
<?php
//<!------------------------ Form action Primary and secondary write to DB--------------------------------------------------->
if (isset($_POST['submit_form']))
    { 
    if ($_POST['air'] != "" && $Air == '' )
        {
        // Set Air variable and then show other input
        $Air = $_POST["air"];
        $GLOBALS['Air'] = $Air;
        }
    if ($_POST["2ndinput"] != "" && $Air != '' )
        {

        //add loop for each check box and write to DB
        foreach($_POST["2ndinput"] as $2ndinput) 
            {
            // Write to DB
            }//end foreach
        echo "<p>Written to the database. Thanks!!</p>";
        }
    } else 
        {
        //set variables to default and reset FORM
        //$Air = "";
        }
?>

Should the Submit action be above or below the FORM in the script? How can I create a variable to manage the FORM action?

Any assistance appreciated. tks