Question: How is window.__data being set by the server (and where?)

I'm looking into storing the redux state in localStorage to be loaded initially, but it seems that the server is already doing this somehow? I'm a little confused as to where the data is setting window.__data and assume its some library call doing it. Thanks!

Additionally we've moved away from sessions and towards a RESTful API but I'm worried about performance in terms of server side rendering / redux model storage and how thats working. I appreciate any advice!