All traders on Bitfinex trade with USD (fiat), not USDT. In line with this, all trading pairs on Bitfinex are placed against USD, not USDT.
USDT on Bitfinex is used as a transport layer, used if a trader wishes to deposit or withdraw in e.g. Omni USDT or Ethereum USDT. Until the trader specifically chooses to transport their fiat in Tether-denominated USD, all their fiat holdings on Bitfinex will be held in the form of fiat USD.
Traders can get both USD and USDT onto Bitfinex through means of direct deposits. Please note that in order to both deposit and withdraw USD (in fiat or USDT denomination), a user will need to be fully KYC-verified.