>Well C is constant
>'c' is constant
NO, it's not. It varies when traveling through different mediums. If you say the "speed of light" is constant traveling through ____-medium at such and such pressure, then you can consider the "constant", which is again relative to a number of other things that can affect it.
https://www.olympus-lifescience.com/en/microscope-resource/primer/lightandcolor/speedoflight/
When it's called a "constant" it refers to the "maximum" "speed of light" in a "vacuum". Which is a whole 'nother debate on numerous hypothetical concepts that don't always apply to reality. Which is why all f this stuff is "relative"…