Poloniex does not support fiat fundng at all.
Then how trading works there?
Poloniex does not support fiat based trading. There are known as altcoin trading platform. You can trade bitcoin against altcoins. Also they are having ethereum and monero demonized altcoin trading (means you can trade select altcoins against ethereum and against monero).
To get you bitcoin trading against fiat, they are having USDT trading. USDT is US dollar tether. It represents USD but only within exchange. You cannot deposit/withdraw it. You can deposit bitcoins to buy tether and then you can trade bitcoins against fiat then you need to buy back bitcoins to withdraw.