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You're confusing a lot of concepts. Let's get this straight. There is the speed of light in a vacuum, it is the same speed of light that cannot be exceeded, because this is the structure of space and time ,время time and space are not absolute and are the same for all observers. And if you measure the speed of any ray of light in a vacuum in its reference frame for any observer, you will get the same value, and other massive bodies will fly at a speed less than light.
In the presence of a substance, the speed of light is less than the speed of light in a vacuum, because light is absorbed and re-emitted by this substance. The speed of movement of individual photons or the speed of propagation of electromagnetic interactions in this case is not equal to the speed of light in a vacuum. Collectively, a set of photons, an electromagnetic wave, travels at a speed less than the speed of light. It is as if the molecules in the air move quickly due to thermal motion, and their collective movement, like wind or sound wave, propagates at a completely different speed.
And the speed of light in a medium can be exceeded, and naturally even while remaining in this medium. Such fast particles or bodies that exceed the speed of light in the medium will emit a specific radiation called Cherenkov radiation (after the physicist Pavel Cherenkov).
Now, as for the speed of light value. The numerical value of the speed of light means nothing fundamentally. Only dimensionless quantities make sense. If the speed of light doubled tomorrow, we wouldn't notice it, because the interactions would change accordingly, and all the distances between atoms and others would also double. As a result, our standard meter would have doubled, and we would have measured the same number. The numerical value of the speed of light is just some kind of scale value that allows us to compare the speed of light with the values in our daily life. All that matters is that it is finite.