Did Einstein ever say how he came up with the fact that the speed of light is always constant .
It's a postulate, not a fact. And not that it is always a constant, but only in a vacuum. I am not aware of any experiment that contradicts this postulate. That's the best you can do with a postulate.
Unfortunately, I cannot answer your question. I don't know how he came up with that idea. He was aware of the failure of some experiments to measure the speed of the earth through the 'luminiferous ether' and that may have had something to do with it.
From the same article.
i see thanks for the responses.
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