In Vim, we can use Ctrl + N or Ctrl + P to finish code completion. How can I configure it in IdeaVim for IntelliJ?
Alt + / and Alt + Shift + / work both in plain IntelliJ and with IdeaVim installed.
Please see the documentation on JetBrains page. It's called Hippie Completion (newer versions call this completion Cyclic Expand Word).
And of course you can always use Ctrl + Space, Ctrl + Shift + Space, Ctrl + Alt + Space.