Unable to recognize Plugin

Below wordpress site is using some plugin:

https://legaldesk.com/documents/rental-agreement
Select State and click create document

However, i have not seen this functionality before. Either they have customized the plugin or have done something

How it is working:

When we click on create document, it shows us a form on the left and previewer on the right. As soon as we fill something in the form, it shows us on the right previewer.

I was bit confused as I am new in wordpress. How they have developed it and synchronized form as well as previewer together.

Thanks