Technology used by sites like unbounce and weebly?

Hi,

If you’re familiar with sites like Weebly and Unbounce, you’ll know that they allow users to login and build web pages and sites.

I wanted to confirm the technology they use for this functionality of letting users build pages and sites (I presume its built in house and combination of javascript and server-side programming language)?

and if there is third party software plugins available which will enable my users with similar functionality?

Hope this makes sense.

Thanks.