The trade happens without price impact, at the oracle's price (provided by Chainlink). The pool self-mints jFIATs at a fixed collateral ratio (CR) that depends on the jFIATs' volatility against USDC.
Whenever a user buys jFIAT, the pool deposits USDC to mint them and sell them; for example, if a user buys for 80 jEUR worth $100, the pool mints them at a 110% CR them by depositing 110 USDC, and then sells them for 100 USDC to the buyer;
whenever a user sells jFIAT, the pool buys them back and burns them to redeem the USDC; for example, if a user sells 80 jEUR worth $100, the pool pays 100 USDC to the seller, and then burns the jEUR to redeem 110 USDC.
From the user perspective, it is a swap between jFIAT and USDC without price impact.