Each time I encounter these turns of phrase, I wonder whether I'm quite grasping the meaning.
Edward Norton says in an interview that
the hardest part was getting it together
as he talks about the filmmaking process.
In Supernatural, Sam says to Dean:
"Dude, get it together."
It's pretty easy to infer what "it" means from context. I am familiar with expressions like pull/get/hold yourself together, get your act together, get your shit together, and so forth.
Is that what the function of "it" is, substituting for all those concepts, as if the speaker couldn't be bothered to use the entire phrase?