How did fuel and ammunition reach amphibious forces?

How were amphibious forces supplied with ammunition and fuel?

Through the wider Atlantic convoy system that carried wartime supplies to overseas forces.

Atlantic convoys carried the fuel and ammunition needed for overseas amphibious operations. That oceanic movement linked landing craft at the beach to the merchant shipping network behind them. The brief answer does not describe unloading, beach distribution, or replenishment ashore, so it covers only the strategic level of the supply chain. Amphibious lift depended on sustained convoy traffic long before the final ship-to-shore movement.