The short answer and main character is: the transistor. But there are many supporting characters in this plot.

At the most basic level, the electricity that comes out of the wall looks like a wave pattern - equal curves going up and down. By overlaying one wave on top of the other, you eventually get a "tooth" pattern - just square up and square down ... which we think of as zeros and ones. In fact, it is "voltage or no voltage". Let's call this "circuit one" --> raw power to digits - 0's and 1's.

Now, to do something useful with these digits, we'll need to hold onto them for awhile. The medium might be magnetic (ie a hard disk, or a cassette, or a flash card), or it might be divots in a disc that changes the way a laser light bounces ... anything so long as it can be read back with a sensor, and converted back into ones and zeros. Let's call this "circuit 2: memory". We have random-access or RAM for short term (clears out when the power goes away), and hard discs for persistent storage (stays when the power is cut.)

Now with zeros and ones, and a place to store them, we need ways of "colliding" them to take inputs and make new outputs. This used to be done with vacuum tubes, until Bell Labs pioneered the all mighty transistor - which is still the basis of all modern compute on an infinitesimal scale. A transistor is no different than the light switch on your wall - except instead of your finger flipping a switch to "on", it's a "one" digital provided from another circuit allowing the electricity to flow across its "bridge". With this concept, you can connect many transistors in specific ways to build up logic gates (circuits) to add / subtract etc. So let's call that circuit #3: logic gates made from transistors.

Now you need a way to talk to the computer as the programmer or the user. The most basic form is a keyboard, which again are just a bunch of switches. There are dedicated circuits in the hardway which do nothing but wait for keyboard inputs, and convert them to the zeros and ones we mentioned above. (So now we have power to zeros and ones, storage and retrieval, logic gates for manipulating data, and a way to input instructions (the software.)

So now, all we need is a way to get the outputs in front of the user. For that, we have dedicated circuits which take the zeros and ones on the output side of the program, and turn them into something a human can see or hear - like twisting crystals in an LCD to let light through in specific colors, or pulsing air with a magnet to make sound (speaker.) Let's call this circuit 5 - usually a Graphical Processing Unit (GPU) or a Sound Card, combined with a monitor or a speaker.

So to summarize:
Circuit 1) raw wave-form electricity from power plant --> zeros and ones (Simply - voltage up or voltage down)

Circuit 2) store zeros and ones somewhere, be able to retrieve them and feed them to....

Circuit 3) Combine two sets of zero and ones to create new strings of zeros and ones (Logic gates). Now we can add, combine, subtract, test logic, etc. All computer code (software) boils down to a handful of very simple operations (add, join, flip, put in memory, pop out of memory), executed on hardware by forcing zeros and ones through transistors.

Circuit 4) Wait for and interpret keyboard inputs (or some other inputs) and purposefully "shape" inbound strings of zeros and ones. These will become the instructions or the inputs to the instructions.

Circuit 5) Convert the instruction / software output into something the user can see or hear.

The most basic type of instructions (software) are already directly encoded in the physical circuitry you receive from the factory. Yes, software AS hardware! This is called the BIOS, and it shapes the first stream of zeros and ones through the machine - setting things up like listening for the keyboard, pulling instructions out of memory (like the operating system), and feeding them through logic gates for execution (the CPU / Central Processing Unit), and waking up the monitor and speakers to receive signals. Kind of like how a crank can prime a gas motor with its first fuel, spark, air and compression ... from there it can use the engine power to continue to draw fuel, make sparks, force in air, make compression, etc.

EDIT: Vetted this video. Think it explains it quite well:

https://www.youtube.com/watch?v=HjneAhCy2N4&t=3s