How does observers travelling at different velocity observe light at same velocity?

According to principle of relativity laws of physics are same for all observers regardless of their velocities at which they are travelling and thus all observers should observe light travelling at 299,792,458 metres per second, regardless of their own velocities at which they are travelling ie light cannot be... show more According to principle of relativity laws of physics are same for all observers regardless of their velocities at which they are travelling and thus all observers should observe light travelling at 299,792,458 metres per second, regardless of their own velocities at which they are travelling ie light cannot be seen at relativistic velocity ,

But how does this phenomenon actually happens ?

Because of which reason even a person travelling at 99.99999999% of velocity of light will also observe light travelling at its original speed ? Please explain in detail or suggest a reliable source .
Update: Please answer this question only if you have logical and to the point , detailed answer
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