'Explaining a Function inside a Function in R

I got confused when I stumbled upon this code as you don't need to give "y" a value to operate, how does it get its value?

f <- function(x) {
    g <- function(y) {
            y + z
    }
    z <- 4
    x + g(x)}

because if I add ran this code

z <- 10
f(3)

it will give the result (10), where the second function got its "y" value?

r function

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