I guess for me, and maybe a some others, I have no idea what you mean by:
just interested in how one uses the HAL libraries with the ARM chips.
What HAL (Hardware Abstraction Layer) libraries?
Yes I know there some CHIP manufactures may define some form of HAL library to use with their chips. And I know that there are some Arduino builds for some chips that may include these as part of their Arduino implementation. For example I know Robotis uses some of the setup including some portions of HAL when they built and released their versions of Arduino for their OpenCM and OpenCR boards.
Some others may choose to have their core files and libraries talk directly to the underlying hardware, without trying to defines some intermediary layer between them. Teensy is more in this camp.
both approaches have their pluses and minuses. For example using some Abstraction layer like some form of HAL, that he chip manufacturer has developed, might allow you to be able to adapt to some new version of a chip, as they probably will release a new version of their HAL that enables the new chip. However the downside, is this often introduces overhead (sometimes a lot), both with memory size as well as speed. Which is always an interesting trade off.
As Paul mentioned, for many of us, it is much easier for us to try to answer things when there are more specific questions.
The Teensy forums are a great place to ask questions. There are lots of us who try to help out as we can. But as Paul mentioned, it is a lot easier to help out, when we have a clue of what information you are actually interested in.
The new T4 is a lot of fun!