How to insert widgets into a post?

You can insert widgets into a post by Shortcode. Just make a shortcode with dynamic_sidebar() function to insert your specified sidebar widgets.

<?php
add_shortcode('sidebar_widgets', function($atts, $content) {
ob_start();
$atts = shortcode_atts(array(
    'sidebar_id' => '',
),$atts);
?>
    <div>
        <?php dynamic_sidebar($atts['sidebar_id']); ?>
    </div>
    <?php
$html = ob_get_clean();
return $html;
});

You can implement this shortcode with any page builder plugin.