Plagiarism / citation standards in non-/less-scientific works
You cannot include citations in speeches; no-one in the audience would stay awake if you tried to do so. And footnotes would be impossible.

The essence of plagiarism is passing off someone else's work as your own. You avoid that in an academic paper by giving full citations. In a speech it is enough to make clear that you are not claiming originality. "As Smith has shown...". "Numerous studies support the view that..." and so forth.