Robinhood Tech Stack?

Hey everyone,

I just came across this site: https://stackshare.io/robinhood/robinhood

Reading Robinhood's page their tech stack is interesting, I was wondering if someone could help explain some of these choices possibly? RxJava? are they combining RxJava and Kotlin for a mobile app? I understand Kafka, Python, Django, Swift. Maybe I understand Kotlin for the mobile side of things, Retrofit is an http client for mobile so I understand that but this seems all over the place too me.

Just wanted to get some thoughts from everyone.