The same place it is in all computers - in RAM.
Samsung, though, doesn't use the clipboard, it uses its own version, that holds more than 1 item at a time. You put things into it by copying or cutting, you take things out of it by pasting. Those are the only accesses you have to it, the same as a RAM-based clipboard. See Belodion's thread,
[GUIDE] Understanding the Clipboard, for more.