Endoscopy light sources consist of a bundle of optical fibers tightly encased in a flexible sheath, illuminated on one end with a light source and polished flat on the other end. Until LEDs became widely available, the illumination source was an incandescent lamp.
As
@kellys_eye said, Google "fiber optics" for information on how "light can flow through a series of tubes." The physics is "total internal reflection" which occurs when light is reflected at the boundary between two substances of different refractive indexes, typically air and plastic or glass. You can see the effect in an aquarium by observing the "mirrored" surface of water underneath from a certain angle through the sides. It is also commonly displayed as a "spray" of
multiple illuminated fibers for decorative effect:
