Can only get value of $_POST['first_name'] from options.php

I created a Menu page in the WordPress Admin that would enable me to add values to the database. These values include things like First Name, Product Title, Product description etc

However I can’t obtain the values through $_POST (except if I add $_POST[‘first_name’] to the options.php page). I can however obtain the value via esc_attr(get_option(‘first_name’)). However I want to be able to do this via the $POST so I can later add these variables to a database

Here is a portion of the code from the relevant pages.

sunset-admin.php

...
$picture = esc_attr(get_option('profile_picture'));
/*$firstName = esc_attr(get_option('first_name'));*/
$lastName = esc_attr(get_option('last_name'));
$fullName = $firstName . ' ' . $lastName;
$description = esc_attr(get_option('user_description'));

$firstName = $_POST['first_name'];
    global $wpdb;
    $table_name = $wpdb->prefix . 'simons_table';

    $wpdb->insert(
        $table_name,
        array(
            'firstname' => $firstName,
            'lastname' => 'Hough',
            'description' => 'This is the description',
        )
    );

?>

<div class="sunset-sidebar-preview">
    <div class="sunset-sidebar">
        <div class="image-container">
<div id="profile-picture-preview" class="profile-picture" style="background-image:url(<?php print $picture; ?>);"></div>
            </div>
        </div>

        <h1 class="sunset-username"><?php print $fullName; ?></h1>
        <h2 class="sunset-description"><?php print $description; ?></h2>
        <div class="icons-wrapper">
        </div>
    </div>
</div>

<form method="post" action="options.php" class="sunset-general-form">
<?php settings_fields('sunset-settings-group'); ?>
<?php do_settings_sections( 'alecaddd-sunset'); ?>
<?php submit_button('Save Changes', 'primary', 'btnSubmit'); ?>
</form>

functions-admin.php :

...
function sunset_sidebar_name() {
    $firstName = esc_attr(get_option('first_name'))
    $firstName = $_POST['first_name'];
    echo '<input type="text" name="first_name" value="'.$firstName.'" placeholder="First Name"/><input type="text" name="last_name" value="'.$lastName.'" placeholder="Last Name"/>';

}

…