He builds the adders from ANDs, ORs and NOTs. With these, you can also build flip-flops, which are memory. (Actually, it's enough to have only NAND (or NOR) to

He builds the adders from ANDs, ORs and NOTs. With these, you can also build flip-flops, which are memory. (Actually, it's enough to have only NAND (or NOR) to build all logic circuits, including memory)