If you're talking about Steemit, it comes from a 9% yearly inflation rate that is decreased by 0.5% yearly until there is 600 million Steem tokens and then there will never be no more made.
As far as fiat goes, it's just extra decimals they punch in their computers at the central banks. From nothing and backed by nothing.
My understanding is the real return of steem = the return - 9%. If I have a lot of steem and don't do anything, then it likely to lose 9% this year. Is it correct?
No, that's incorrect. Steem currently operates at a 9.5% inflation rate, meaning, 9.5% more Steem tokens will be created this year based off the current Steem tokens in circulation. For example, let's say there is $100 US dollars currently in circulation around the world. If we produced more US dollars at a 9.5% inflation rate this year, we would create an additional $9.50 of US dollars(9.5% of $100), now bringing the total to $109.50 US dollars. Steem tokens will continue to be created like this, decreasing the inflation rate by 0.5% yearly, so in 19 years, there will be no more Steem Tokens to be made.
You actually earn 2% interest by holding your Steem in Steem Power. Just another way you can accumulate Steem.