On FTX the tradable balance is based on a collateral pool based on a bunch of different currencies. All margin is posted in 'USD' in your wallet. USD can be funded by depositing either USDC, TUSD, PAX, BUSD, or HUSD.
By default all positions use the same collateral pool, and all USD, non-USD fiat, BTC, USDT, ETH, BNB, PAXG, XAUT, BCH, LTC, LINK, UNI, TRYB, KNC, SRM, FTT, CUSDT, XRP, SOL, BVOL, IBVOL, and TRX in your wallet count as collateral. Each subaccount has one central collateral wallet and uses cross margining for the account. Each subaccount has separate margin and collateral from other subaccounts.