I am having trouble understanding degree-to-radian measuring. In…

I am having trouble understanding degree-to-radian measuring. In this not here, I don’t understand, what they are trying to establish:
I don’t understand what they mean my theta over 360= r/ 2pi r
I know that 360 is the circumference of a circle, but I don’t understand the relationship between theta and r is.
Another thing related to radians that i dont understand is:
a/r is the arc length over radius. Which is something that also confuses me because above it was written as 2pi r/ r. Why is there a difference? As well, here, why are they using multiplying by 2pi here?
I belive this is very simple thing to answer, i am just confused with the correlation of everything.

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