In Corey Schafer's Programming Terms: Mutable vs Immutable, at 3:06, he selected multiple lines and commented them out in PyCharm all in one action.
What is this action? Is it a built-in shortcut in PyCharm that I can use or configure myself?
This is a setting you can change and define in "Settings".
The default is with Ctrl+/ for Windows, or Cmd+/ for Mac.