I was a 12 year old when I learned about them :)
The idea of a pixel is that it's the smallest area of the screen that can be independently controlled, thus the designation "pixel" for "picture element".
In most modern phone screens or monitors, each pixel is formed by a group of three smaller elements with fixed colors (usually red, green, and blue) but _variable brightness_. By controlling the brightness of these sub-elements, we can control what the overall color of the pixel appears to be - once you get more than an inch or two away from the screen, the light from the element group blends into what we see as a single color - so 100% green + 100% red + 0% blue looks like bright yellow. 50% each for red, green, and blue looks like a middling grey. You can usually see the structure of the pixel with a magnifying glass, though this is easier with an old TV or monitor than a modern phone.
These pixels are laid out on a regular rectangular grid, and your display controller will offer some way to set the color of each pixel and to then update them all at some (usually) regular interval, for example 60 times per second. In computers, it's common to keep a "frame buffer" around that stores separate values for the red, green, and blue components of every pixel on the screen. If these are 8-bit values, that implies that each R/G/B component can have 256 different levels of brightness, and in combination they allow each pixel to take on one of about 16,000,000 possible colors. So, a program can change pixel colors by writing different values into this buffer and waiting for the updated buffer to be processed by the display controller to change what the display is showing.
Of course, the electronics, physics, chemistry, timing, and logic of display generation have changed quite a bit since Pong. And not all displays even have pixels - vector displays used to be a thing, and are still used in some very specialized applications.