NAND and NOR gates are called digital building blocks because they are fundamental building blocks for digital circuits. Both NAND and NOR gates are basic logic gates that can be used to construct a wide range of digital circuits, including more complex logic gates and digital systems.
NAND and NOR gates are called "universal gates" because they can be used to construct any other logic gates by connecting them in specific ways. For example, by connecting NAND gates in series or parallel, you can create NOT, AND, OR, and XOR gates. Similarly, by connecting NOR gates in series or parallel, you can create NOT, AND, OR, and XOR gates as well.
These gates are also called "building blocks" because they can be combined and used to create more complex circuits such as counters, flip-flops, digital multiplexers, and digital encoders. By connecting these gates together in different ways, you can create a wide range of digital circuits that perform different functions and can be used in a variety of applications.
In summary, NAND and NOR gates are called digital building blocks because they are fundamental building blocks for digital circuits, they are universal gates that can be used to construct any other logic gates and they can be combined and used to create more complex circuits.