This Statamic blog post has (at least some) insights:
https://statamic.com/blog/statamic-at-spiegel-scale-2> So how does SPIEGEL do it?
> Curious about how SPIEGEL’s stack works? Here’s their high level approach, keeping in mind this is running on v2. Upgrading to v3 streamlines even more of their stack.
> A huge flat file content store organized with each entry inside its own subdirectory (you can’t have more than 10k files inside a single directory, an operating system limitation)
> The content store is using a cloud storage solution.
> Statamic’s control panel pushes entries into the content store, and then into an ElasticSearch instance via message queue.
> ElasticSearch runs as a content API with blazing fast response times and is consumed by the control panel and front-end
> The front-end is built in Go.
> User accounts are connected with an Office 365 Active Directory integration with OAuth.