I had the same question...
Coming from the adafruit world and Circuitpython, I'm used to having a reference like this https://docs.circuitpython.org/en/latest/shared-bindings/index.html (which is very comprehensive and a lot of work)

UIFlow does seem pretty easy to understand, even without documentation.
But when I do something in micropython like

import unit

What am I actually importing? is source code available?