With software which speaks to hardware. Let's say your program says PRINT "a" The program will send the ASCII code for "a" to the operating system, saying this is to be printed. The operating system will look up the pattern of pixels for "a" in a font file, and send it to the video card. The video card will store it in the frame buffer, which is then repeatedly read and sent to the screen, which will decode the pixel data to decide which dots of red, green and blue should be illuminated.

This is a bit simplified, because i don't actually know any of this in detail, but it shows the principle.

The great thing is that all the links in the chain only need to understand its immediate neighbors. The screen doesn't need to understand a frame buffer, or an OS, it just needs to translate a video signal to a screen image. And the programmer doesn't need to know anything about how the screen works, just how to tell the computer to display something. This divides the immense complexity of modern computers into components that a team of engineers can comprehend.