Are you talking about there being support for multiple language libraries like PyArrow or about there being multiple Apache projects that utilize Arrow like Parquet and Spark?

If not, I'm not following what sub-projects you are speaking about. As far as I know, Arrow is principally the Arrow Columnar Format and Arrow Flight with some other potentially interesting interfaces for compute kernels and CUDA devices.

Am I missing something?

In general, where's the best place to learn more about Arrow? I've approached it several times, and can find a lot about how to integrate it into other products, but none of the tools like the query engines that I would find very useful.