UniswapX outsources routing complexity to an open network of third-party fillers.
Fillers then compete to fill swaps using onchain liquidity (from Uniswap v2, v3, and v4) or tokens from their own private inventory.
Anyone can become a third party filler for UniswapX swaps. See here for more information on becoming a filler: https://docs.uniswap.org/contracts/uniswapx/overview