I think you're comparing oranges to apples. The same meter that measures 10A DC will not measure 1000A AC. Current is a series measurement. 1000A AC would fry most anything that does not have a cable that is less than a half inch in diameter.

For measuring higher currents you simply use a high wattage, low tolerance, 1 Ohm resistor wired in series with the load. You then take a voltage measurement across the resistor and using Ohms law determine the current through the resistor. Inside the DMM, that's what they are doing, it's just that they can't fit a very high wattage resistor into the case and provide large enough wires to handle the current.

Have a great day,