How is Residual Standard Error (RSE) calculate within R's lm function?

Question

I performed linear regression in the following way:

df <- data.frame(y = c(1,2,3,4,5,6,7,8), y1 = c(1.05,2.1,3.5,3.9,4.8,6.1,6.8,7))
mod <- lm(y ~ ., data =