I have one advise: look up a good course on special relativity (there is a chapter on it in the feyman lectures if you don’t know were to start). Although it is hard to really understant, it follows from the statement that any observer sees light moving at the same velocity (comes from experiment) and some basic math.
We think it’s weird because we are not used to it. We live in a world were everything we can see moves at slow velocities (compared to light). It is hard to give an eli5 besides every experiment we have done afterwards says it is this way.
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