What did ancient philosophers, like Plato, use to write their works on/with, physically? (Tree bark, animal skin, types of writing utensils, etc)
Nobody knows what Plato wrote on because no originals survive.
However, papyrus was the standard at the time of Plato. You can check out some examples here. Leather was also used for writing in his time, but parchment probably wasn't popular until at least couple hundred years later.
It's worth noting that parchment wasn't simply invented on a particular date. It was gradually developed in stages over a long period of time. Different treatments of animal skin were used for writing and had been for thousands of years before Plato. There's a good chance ancient Greeks, Plato included, had access to something like parchment.
According to Quintilian (Inst. 8 6.64) and Diogenes Laertius (3.37), Plato probably first wrote on wax tablets, and then retranscribed them on papyrus. The image of the wax tablet (Plato: Theaetetus, Timaeus. Aristotle:De Anima, Parva Naturalia) to illustrate the memory, the acquisition of knowledge or the nature of intellect is also a sign of this practice.