But you cannot really multiply the crypto. You see, if you have 10 dollars in deposits all you need to do to create 30 dollars is make a note on an account. You cannot create crypto out of nothing like banks do.
Imagine this situation - whatever_coin have a total supply of 100 coins. You're the owner of the whole 100 and you are depositing these coins to 'coinbank'.
Now this bank would be able to lend me 100 whatever_coins (which I'd like to spend on some services you provide)
So now you have 100 your initial coins and 100 coins on your deposit.