I deposit a dollar in a bank account somewhere and mint one DollarCoin. The crypto market cap is $1.
Repeat that several times and add lending.
Repeat that several times and add lending.
Now apparently we have just created 1$ out of thin air.
I like that example as it already shows one of the fundamental risks of crypto.
And now you can't even just say BTC or eth or stable coin because this is true for all.
As long as no one tries to cash it into real money the BTC price goes up and so does the market cap