1. Kotlin is basically modern Java, and is therefore an obvious choice for Android which still runs everything in a JVM.
2. Go was basically designed to run network services and it is very good at it. The standard library is excellent (probably the best of any language) and concurrency is pretty easy too.
3. The Flutter team have published their reasoning for using Dart. Can't say I fully agree with all of them but it is a reasonably nice language (way better than Javascript anyway - although I know that doesn't take much) and developing with it is really pleasant.
I don't know why TF chose Swift though. It looks like a nice enough language though.
There are definitely significant differences between all of those languages. It's not like they're using a mixture of Javascript, Typescript, Flow and Dart. That would be silly.