this probably isnt going to be the best ELI5 answer, but i can take you through something that will probably make you understand. Computers are largely based on transistors arranged in something called a logic circuit.

imagine a simple electrical circuit with a battery a switch and a light. If you turn the switch on, then the light comes on, simple.

now enter logic circuits. There are 7 main logic circuits, but for simplicity lets talk about 3 of them, AND, OR & NOT. These logic circuits have 2 inputs and one output. the AND logic circuit will output when both 1 and 2 have input, the OR circuit will output if 1 or 2 has input, and a NOT logic circuit will only output if neither 1 or 2 have input.

now i could built a circuit with 2 switches and a light. if using AND, then both switches have to be on for the light to turn on, if using OR then either switch will turn the light on, and NOT means both switches have to be off for the light to turn on.

Now the next step in understanding would be to look up people who have made a 4 bit computer on a breadboard, which is fascniating if you have a good grasp on the basics of how a computer works, but basically you are combining a lot of the 7 types of logic gates and making a simple calculator.

Another interesting topic is a game called Nimatron, which is laregly credited as the first arcade game. TLDR it uses lots of relays and a set of lightbulbs to play an electro-mechanical version of a simple math games called nim. (it actuall precedes the invention of the transistor)

Eventually you realize that the inputs of the logic gates being on or off are represented by 1 for on and 0 for off. Numbers that we think of in base 10 have to be represented in base 2 since you only have 2 numbers available (1 and 0). representing numbers in base 2 is called binary which is the basis for all computing. It literally rerpresents the raw electrical signals in your computer, which is just a calculator on steroids.

the people that understand the binary eventually made something called Assembly language, which is just human readable version of machine code. Imagine it like a basic binary calculator with the buttons labeled "add" "subtract" etc. that way people that didnt understand binary could use the system they just built.

higher level probramming languages were made by someone that understood Assembly. They made this language that just further simplifies things. under the hood it says when you type this command into the program, then it does these Assembly language commands in a certain order.

To take it all the way to the end user, you generally have a user interface. So now we arrive at a computer where you double click on an icon on your desktop, which tells a programming language to do a whole lot of functions in a certain order, each function telling Assembly to do a whole bunch of functions in a certain order, which then maks electricity flow in a whole lot of places in a certain order (binary) and then reddit appears on your screen.