What if all a computer could do was add?

Is there even a way to envision this? Now is the time for some bright minds to enter this discussion of how computers work. We’ve got quantum computers about to emerge with real world tasks this year. It’s high time we at least knew how the current computer works.

Don’t be shy. Articles and other discourses are welcome, so long as they are not spam.

For openers, somebody please explain NAND gates. Either a logic or a circuit explanation would be appropriate.