They use several custom plugins that they’ve developed over the past several years. They have a bunch of things that I think you don’t need, like a resume builder and some other things that I’ve not paid close attention to.
If you say you want a site like theirs, then you need several years and tens of thousands in development costs.
But you don’t really want a site like theirs. Their’s doesn’t have any support for parents that I’m aware of. The first thing to do would be to identify exactly what things that they have that a stock Discourse site does not have and then look for how to do that.
Then you can see what those changes are worth to you and then look for a solution that fits your budget. When you’ve done that you can contact me or post in Marketplace.