$EURT

Been dabbling more with stablecoin payments lately, mostly small stuff, but I'm trying to wrap my head around scaling it. When fintechs or merchants are moving serious volume, say in $USDC or $EURT, how do they manage the on/off-ramp liquidity without getting absolutely hammered by slippage, especially on larger, less common fiat pairs? Are there dedicated OTC desks for this, or more sophisticated liquidity pools I'm not aware of?