download icon disappeared from toolbar

Choose View > Customize Toolbar, then re-add the item:

Customize the browser window - Apple Support

I think it shows even when nothing is downloading, but when some previously downloaded file is still in the downloads folder. At least mine does.

I guess it is possible the OP is asking about the download folder in the Dock. If so, it must be dragged from your home folder to the dock and an icon will be created there.

If you are talking about Safari, it only shows if there is an active download.

Try Safari/Preferences/General/Remove download list items - set to manually.

I think it shows even when nothing is downloading, but when some previously downloaded file is still in the downloads folder. At least mine does.

I have several files in my Downloads folder, but it doesn't show. It may disappear when you have processed the download (open, move, etc.).

You may be correct about the Dock.