I saw a video, it was about a guy that does multiplication in his mind.

[RandomThoughts]

While he did it, he moved his hand around on a table.

About an hour later I saw a video on only being able to think of only one thing at a time.

That is not true, you can think of multiple things at once by using different areas of the brain.

The way that guy was doing the multiplication in his head was he was storing digits to use later in his finger memory area. So he really was doing multiplication just like on paper, but he has areas of physical movement that he has learned to store numbers in.

I use to do some multiplication in my head, and used to store one set of numbers in visual memory, another in hearing, and another in speech, so you can think of multiple things at once. Then while doing a computation, you store some number as a visual image, store another as the sound of that word, and another as the repeated pattern of saying some number. Then use your computation part to do some math, and retrieve or store numbers just like doing math on paper.

And many parts of the mind can hold communications or stored information, like dance, or tone, or anything that can be perceived, can be ordered to hold a set of information.