So how does this thing grow you may ask, well let me tell you how to sustain a blockchain there must be something called a ledger where all the transaction has to be noted and to become a block in a blockchain a hashing function has to be solved and producing bitcoin by solving hashes are called Mining Bitcoin.
To solve the hashing function typically people used graphics cards but as time passed by bitcoin got more difficult to mine and dedicated hardware called ASIC miners were introduced.
So people who spend their time and money to mine bitcoin will be rewarded in bitcoins and as more and more people become part of this blockchain its value will increase.
so how can I make money with bitcoin, there are two ways
1.you can trade bitcoin/altcoin(any cryptocurrency other than bitcoin is called altcoin, yes there are thousands of them)
2.you can mine them on your own