How to edit contents of dynamic_sidebar()?

I know PHP well but I’m new to WordPress. I got an incomplete WordPress website. It has a static home page with an Aweber newsletter form. I tried digging into installed theme and in home.php I came across:

<div class="home-infobox">
<?php if ( is_active_sidebar( 'homepage-infobox' ) ) : ?>
    <?php dynamic_sidebar( 'homepage-infobox' ); ?>
<?php endif; ?>
</div>

Now when I view source on the home page, the Aweber newsletter subscription form appears exactly within <div class="home-infobox">. I have to make changes to the form as my first step and I tried to figure out from where is it fetching the content? Is it from the database or from any PHP files, I am totally clueless.
Please help me figure this out.

1. The contents of dynamic_sidebar are pulled from the widgets associated with this “Sidebar” aka “Widget Area” in wp-admin, as @s_ha_dum answered. There is no template file for the sidebar itself. Visit /wp-admin/widgets.php under Appearance -> Widgets and find the widget area titled homepage-infobox. You be able to add/remove widgets and possibly make changes to the content here.
2. Search your plugin and theme files, possibly functions.php for register_sidebar to find one using the slug homepage-infobox. The declaration of register_sidebar may include some presentation code.

See also

• https://developer.wordpress.org/themes/functionality/sidebars/
• https://codex.wordpress.org/Function_Reference/dynamic_sidebar