string.toCharArray() underneath makes a copy of the String character array (defensive programming)
If we prepare array by making new char[string.lenth()] and then iterate trought string using chatAt(i) we could avoid one array copy

string + string in a loop = multiple stringbuilders => bad
1 stringbuider (appending in a loop) => good

source? since what version?

I have been told that String use StringBuilder in higher version, so it isn't necessary anymore.

Definitely true, but in case of few number of iterations, like in the cata above, you won't notice a difference in time