Where do I find the .csv file for the submission after creating it?
When you are done with Colab instince files saves will not persist. You have to configure saving them to google drive. Try something like this; 


  from google.colab import drive
  drive.mount('/content/gdrive', force_remount=True)
  root_dir = "/content/gdrive/My Drive/"
  base_dir = root_dir + 'fastai-v3/'


so now you can access and save files to google drive directly.