In piCore it is the Unix version of MicroPython with os module preinstalled, see /usr/lib/micropython. There is a piCore specific tc.py module also with functions used by the extension loader like reading of kernel version, number of CPU's, dependency lists, etc.
At the moment there are no RPi machine hardware (HAL) module available, someone must make it. As much as possible it must follow the rules I mean use same methods, properties, etc. as other MicroPython implementations are doing it. Which is changing of course, see ongoing discussion at GitHub about that topic.
Many hw stuff can be done via /proc like configuring, reading and writing GPIO, etc. It is easy to code as it requires only file manipulation. Direct programing of hw registers a bit more complicated and requires a C code as well as using interrupts. But you can start with just the simple operations. There are tons of sample codes for RPi written in other languages.
As cPython MicropYthon on Linux is using libffi which makes using of existing libs easy. Study MicroPython modules at GitHub.