Where do I create a create_file_dest file for a pdb?

I’m trying to learn how to create a PDB from PDB$SEED and I just have one question about this video that has been great at spoon feeding me information. What program is this guy using to mkdir for his create_file_dest file? Also, where is he creating this file? Or more importantly, where should one create this file and how do I know that i’m in the right place? He just exits and creates a directory.. but where?

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Author: ellie-lumen