How can I put download file back on dock?

Open a Finder window, select the Go menu (on the menu bar) > Go to Folder, and type ~. Then, drag Downloads to the right part of the Dock.

Another option is to go to the Finder menu (on the menu bar) > Preferences > Sidebar, and mark Downloads, so the Downloads folder will appear on the Finder sidebar

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If you want to put a folder in the Dock, open a Finder window and drag the folder to the right part of the Dock. In this case, you want to put the Downloads folder in the Dock again, so open a Finder window and drag Downloads from the Finder sidebar to the right part of the Dock

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