Thanks for writing in!
While there's lot that goes in Masonry Layout under the hood. However, you can take a look at following file:
_index.php under /wp-content/themes/pro/framework/views/global/.
Before making any changes, please make sure to install and setup child theme. Please refer following resources to get started.
You can use following resources to download and setup child theme.
Download Child theme from following source: https://theme.co/apex/child-themes
Please take a look at following article to setup child theme: