The way CD burners work is complicated. But first you have to know what a CD is.
A CD is a mainly a piece of injection molded of clear polycarbonate plastic that is about 4/100’s of an inch. Then while the CD is being made it is implanted with tiny bumps arranged in a spiral track that data is put in. Then a thin aluminum layer is put on the disk. The label is then put on the acrylic. So it then looks like this
And in these bumps the data is stored. Also due to the reflective aluminum cover the surface of the CD is a prefect mirror that the bumps then disrupt in order that the laser reads these imperfections by the difference of the reflections.
CD burners start by the CD fabrication machine using a high powered laser to etch the bump pattern into a photoresist material. Through the imprinting process this pattern is pressed onto the acrylic discs. The disc is then coated with an aluminum material to make a readable reflective surface. Finally the disc is coated with a plastic surface that protects the reflective metal. When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can't pass through. By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern. The light from the player's laser beam will only bounce back to the sensor when the dye is left translucent, in the same way that it will only bounce back from the flat areas of a conventional CD. So, even though the CD-R disc doesn't have any bumps pressed into it at all, it behaves just like a standard disc.
So now that you know some of the background information:
the CD burner has a moving laser assembly. With both a “read” and a “write” laser, and the “write” laser alters the surface of the CD-R, instead of bouncing light off it. Then while the disc spins in the CD burner the laser moves from the center to the outside as the laser spins. The bottom plastic layer has grooves that guide the laser a long the correct path of it. To record the data, the burner simply turns the laser writer on and off in synch with the pattern of 1s and 0s. The laser darkens the material to encode a 0 and leaves it translucent to encode a 1. So finally you have a burned CD.