Why?

random test cases

8 + 8 = 16, 16 is a two digit number... so you have to repeat the process. 1 + 6 = 7. 7 is not even.

Which part exactly?
Reading this solution I learnt that one could aggregate lists with a "+":
["a", "b"] + ["c", "d"] = ["a", "b", "c", "d"]
Does this help understanding?

Thanks, I got it because of your explanation