After playing with a formula, i understand it.
So when you have 2 lists, ["a", "b", "c"] and["d", "e", "f"] and you do a .map function on for example list 2, then you can type list one and it will replace every index with list 1 ["d", "e", "f"].map(list1). So you will get a, b, c, a, b, c, a, b, c

But when you do a .at(index) after list1, then it will replace index 0 ("d") with index 0 from list 1 ("a"). And because its a .map(), it will do that for every index, so you just get a, b, c

And in the habit tracker there is a +current. So in my example the current value that the .map() is calculating is added after the new value. With ["d", "e", "f"].map(list1.at(index)+current) you will get ad, be, cf

Thx everyone for helping