Autocorrect on an iPhone converts three periods to an ellipsis.

. . . => …

Windows: Alt+0133 - numbers typed with the number pad. The character map app on Windows can help find all of these codes for different glyphs.

On iOS, you can also long press the period button, then select the ellipsis in the pop up menu for accented characters and diacritics.

on macOS, option + ; seems to work …