Had a contractor put in a new walk in shower. Currently the shower “arm” is just a plain black pipe with a cap on the end. Am I able to buy any shower head/arm for this? I’d imagine they will cut off and re-thread? Just not sure how it’s done.

Get a shower arm and head from Big box. Make sure your water is off and remove the pipe with the black cap and install your new arm and shower head....done.

srloren

I haven't tried to remove the black pipe with cap thats sticking out, but it seems like its pretty solid in the wall. Does it just unscrew?

the fact that is is solidly attached is a very good sign.
yes, it does just unscrew. counter clockwise is the direction you want to turn.
if you are picking up a new arm, flange and head then also get a small roll of teflon tape to put on the male threads of the new arm.
if you need further assistance for installing the new set-up... just ask.

Have already purchased the arm and head. Have the tape as well. Just wanted to be sure there wasn't another step I was missing with the pipe sticking out and not being able to screw anything into it. Sounds like a pipe wrench will loosen it up and I'll be able to install everything just fine!

when you remove the black nipple from the wall, take note of how far behind the wall the threaded elbow is.
the new shower arm has different length ends. one longer than the other.
just make sure the bend in the new arm is not too close to the wall or the flange will not slide up enough to cover the hole.

All the answers above are correct, but why didn't the contractor come back to trim it?