[cfitz]

Yup, just open Preview for an image (possibly other documents as well e.g., PDF), go into “markup” mode, and begin to draw with the pen or pencil. Preview will try to “perfect” your drawn check mark, X, straight line, curved line, etc.

[herodotus]

That's a nice feature! But it was added after we left Apple. I cannot claim any credit for it.

[coldtea]

That's a different "smart" feature -- the one mentioned has to do with guessing where the selected text continues (when it goes on to a different column, text box, etc).

PDFs are more "baked" layouts rather than high level semantic markup, so it's not easy to automatically determine this. The AI is probably a set of heuristics.