Can you download more animated overlays/elements?

You cannot use an mp4 video file as an overlay in Photoshop Elements, but you may be able to do so in Premiere Elements, if the file has an alpha (transparency) channel.  (The Elements animated overlays can also be saved as mp4 files.)  As I have already indicated, an animated overlay consists of a series of frames (video) or layers (gif).  Each frame or layer has a subject, such as falling snow or leaves or bursting fireworks.   This subject has a transparent background through which the underlying image shows through.  Each frame or layer changes the position of the subject, and when they are displayed in quick succession, they have the appearance of movement or animation.  Here is a screenshot of the Elements blue butterfly overlay on a white background.  It comprises over 100 layers.

Each layer, originally starts with the background image and another layer sitting over it on a transparent layer.  The two layers are then flattened or combined.  A new set of layers is then created, using the background image and the transparent ovelay - with the overlay moved or changed e.g. the autumn leaf drops down and/or turns.  This second set of layers is then flattened or combined.  And so on and so on . . . .

I don't know what else to tell you without you being more specific as to what you want to do or create.