How are light-years measured

 

Response to comment [from a Christian]: "...[T]he product of the speed of light..."

 

Right.

"Einstein recalled how, at the age of 16, he imagined chasing after a beam of light and that the thought experiment had played a memorable role in his development of special relativity..."
Chasing a Beam of Light: Einstein's Most Famous Thought Experiment

 

"...What about the speed of light, how fast it travels?"

 

It's God's speed limit.

Light is fascinating (Gen. 1:3–5).

See:

Let there be Light (right click, open) by Daryl Ferguson
 

How are light-years measured