I am developing a custom WordPress theme, and in the admin menu, I have a link called Theme Settings. Inside theme settings, I created a side menu, and on the right side, I want to display the content. How can I tackle this with PHP?

<style src="<?=get_template_directory_uri();?>/css/flex-box.min.css"></style>
<script type="text/javascript" src="<?=get_template_directory_uri();?>/js/admin/footer.js"></script>

<div class="container-fluid">
    <h1>Project Forty Theme</h1>
    <div class="row">
        <div class="col-xs-12 col-sm col-md-3 col-lg-3">
            <nav>
                <ul>
                    <li><a href="<?=get_template_directory_uri();?>/inc/admin/theme-parts/header.php">Header Settings</a></li>
                    <li><a href="<?=get_template_directory_uri();?>/inc/admin/theme-parts/home.php">Footer Settings</a></li>
                </ul>
            </nav>
        </div>
        <div class="col-xs-12 col-sm col-md-9 col-lg-9">
            Content Display here
        </div>
    </div>
</div>

Theme Settings Page