Circuitverse.org is an an online digital logic simulator that allows you to build digital circuits in your browser.
I will be coming back to this later, but I needed somewhere to host an image.
There are two circuits, the top right one is showing a plain NAND gate, the bottom one builds an OR gate from three NAND gates.
This is for the FutureLearn How Computers Work course, from the Raspberry Pi Foundation.