Answer 1

Java can be used for ML as well but more libraries exist in Python because Python is very popular and easier to write. Especially in scientific environment and research it is more popular and there a language like Java doesn't make sense. If you need fast execution you would rather use C/C++ or Python is the better choice.

The platform doesn't matter because you can easily run Python on any platform too. Usually it runs on a research or test machine or server. Especially a server has Java only if Java is really required.