I poked around in their documentation and found a sample API endpoint (https://developers.openweb.com/docs/enable-search-engines-to-index-your-content) that I hit and inspected the result of.
The response isn't super telling, but it appears they're utilizing "fasthttp" as their server engine, which is a module in the Revel framework written in Go. https://revel.github.io
I don't have anything to say about their frontend since I didn't dig deep enough to find a live implementation, but nowadays, it's probably a safe assumption it's using something like React, Angular, Vue, or some other JavaScript framework.