I used HTTRack and downloaded the whole site to move the Assets to another site. Otherwise you have to do it one at a time. Sames as deleting, one at a time.

Assets is a bit limited and I preferred the files tool.

The major challenge is when you delete a page or pages the assets hang around and it's a pin to delete them.

More so when you don't use foolders.

Be nice to delete folders or deletion of several files, especially when the content get's old.