Shortcode to show a grid of all demo sites with featured image in the network.

demo_sites_grid_shortcode.php

<?php
/**
 * Shortcode to show a grid of all demo sites with featured image in the network.
 *
 */
function demo_sites_grid_shortcode( ) {
	ob_start();
	
	$subsites = get_sites();
	if ( ! empty ( $subsites ) ) {	
		echo '<div class="wp-block-columns">';
		foreach( $subsites as $subsite ) {
			$subsite_id = get_object_vars( $subsite )["blog_id"];
			if ( $subsite_id === '1' ) {
				continue;
			}
			switch_to_blog( $subsite_id );
			$frontpage_id = get_option( 'page_on_front' );
			if ( has_post_thumbnail( $frontpage_id ) ) { ?>
				<div class="wp-block-column" style="flex-basis:50%">
					<a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php bloginfo( 'name' ); ?>">
					<?php
						echo get_the_post_thumbnail( $frontpage_id, 'large' );
						echo '<p class="text-center">' . get_post_meta( get_post_thumbnail_id( $frontpage_id ), '_wp_attachment_image_alt', true ) . '</p>';
					?>
					</a>
				</div>  <!-- end wp-block-column -->
				<?php
			}
			restore_current_blog();
		}
		echo '</div> <!-- end wp-block-columns -->';
	}
	$temp_content = ob_get_contents();
	ob_end_clean();
	return $temp_content;
}
add_shortcode( 'demo_sites_grid', 'demo_sites_grid_shortcode' );