"What are the Goo parts?"

The Goo custom parts are assets built by VoidBeat that are meant to be basic building blocks to attend a variety of creature shapes in game, hopefully expanding possibilities of creation in game.
Parts such as "Skull Craneum" and "Cotton Ball" were direct inspirations by how often they are used to build creatures bodies/limbs and other details.

While true you can use one part of each "Goo Part", they work often better in conjunction.
They are called "Goo" by how squishy, stretchy they are.

Check Out my Other Self Built Parts here!
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3560475403&savesuccess=1

Side note: PARTS IN MULTIPLAYER DOES NOT WORK DUE TO GAME BUG.
However, you can still build with them in Singleplayer
This observation is relevant as long the game still has it.