Out of curiosity I just thought where do bitcoins came from??

Person A deposits $100 in a bank, which means he has $100 he can spend. In accordance with Federal Reserve regulations, the bank is allowed to loan up to 90% of this money to someone else, say, Person B.

Once Person B has $90 in his bank account, this makes a total of $190 available to be spent. But his bank can loan up to $81 (90% of $90) to someone else. Now a total of $271 is available to be spent. And so on.