When passing a Ruby array as an argument, why does `<<` append while `+=` does not?

Question

I ran across an unexpected result when passing an array to a function and the behavior of << vs +=.

Can anyone explain why the following

Answer 1

My understanding is that in Ruby all function arguments are passed by reference

No, as you have discovered yourself, Ruby is pass-by-value, not pass-by-reference. That's why you are seeing the result you are seeing.