A computer is, basically, a fancy calculator. You can make a very simple calculator with marbles and some wood: https://woodgears.ca/marbleadd/.

If you make the little planks switch direction with electricity instead, and you also replace the marbles with electrical paths, you've basically got a vacuum tube. This sort of lets you do the same thing the marbles are doing, but faster.

Vacuum tubes are big and expensive, but transistors, which do the same thing but are way smaller, are a great alternative.

If you take a block of silicon (the rock) and slice it into a thin wafer, it turns out you can dig little trenches into them and make A LOT of transistors, and you can similarly wire them all together however you like. This is a horrendously complicated process, but you're left with something kind of like the marble machine except with literally billions of switches.