my canon mg3620 printed pages have random ink smudges not from the document, how do i clean it?

Ink inside the printer, on the bottom plate. Perform a bottom plate cleaning.

From the printer:

1. Power on. Remove all paper.
2. Fold a new A4 or Letter plain paper sheet in half widthwise. Unfold.
3. Fold one side to the center crease. Unfold.
4. Load this single sheet, crease ridges up, the edge of the uncreased half facing the far side.
5. Open output tray extension, open paper output tray, extend support.
6. Hold Stop button until Alarm lamp flashes eight times. Release.

The paper feeds through and cleans the interior. Check the ejected paper's creases. If smudged, repeat.

From your computer, use the driver: Open printer setup, Maintenance tab, click Bottom Plate Cleaning. Follow the on-screen instructions to fold and load the paper, then click Execute.

Do this regularly to prevent smudging. If smudges persist after cleaning, there may be a leak or other fault.