It helps if you click the link of the operator(s) under TinyAPL. For example Blackbird goes to https://tinyapl.rubenverg.com/docs/primitive/atop

Which seems to be function composition and some extra rules about if there's 1 or 2 arguments, so the 2 arguments go to G and F is applied as a single argument function. Anyway if you click on a few of these an look at the red text in the upper right it's fairly clear what it's doing.