Which cover file should I use for mockups?

Use the cover you built in the Cover Maker — load it straight from the editor, or open the book's project file. The mockup tool then renders your cover as a realistic 3D object, so what you see is your actual book, not a generic template.

Because it pulls from your project, it also knows your trim size and page count, which set the book's real dimensions and spine thickness. See Are the mockups based on my actual trim size and spine width?

The Social Pack renders from the same project and the same 3D book, so anything true of your mockups is true of your videos — see Can I make a video of my book?