They aren’t. What you’ve seen is called a frontlight.
E-ink panels are not transparent, light cannot pass through them at all. The complete opacity of the ink particles inside the panel is part of how it works. They physically cannot be backlit.
The built in light you find on most new e-readers is a string of LEDs around the edges of the panel, which shine light into a glass or plastic layer in front of the screen.
This allows the device to illuminate the e-ink panel in exactly the same manner ambient light does.
A lot of them are, but you can turn it off when there’s enough ambient light.
They aren’t. What you’ve seen is called a frontlight.
E-ink panels are not transparent, light cannot pass through them at all. The complete opacity of the ink particles inside the panel is part of how it works. They physically cannot be backlit.
The built in light you find on most new e-readers is a string of LEDs around the edges of the panel, which shine light into a glass or plastic layer in front of the screen.
This allows the device to illuminate the e-ink panel in exactly the same manner ambient light does.