How do you add multiple design templates for Powerpoint?

I'm not sure how to use the Slide Master, but all I basically need to copy and paste different slides from different files, which ALL have different design templates. But when all combined into one presentation, the slides all change to one design template. So how do I have slides with different design templates in one presentation|||Copy and paste the slides from presentation A into presentation B. Immediately after pasting them, look near the right side of the last slide you pasted (in the slide sorter or slide miniatures of work view) for a tiny clipboard icon. Pull down the arrow and select Keep Source Formatting. That's all there is to it.