String interpolation is an implicit to_s call. So, something like this:
result = "hello #{expr}"
is more or less equivalent to this:
result = "hello " + expr.to_s
As karim79 said, a symbol is not a string but symbols do have to_s methods so your interpolation works; your attempt at using + for concatenation doesn’t work because there is no implementation of + available that understand a string on the left side and a symbol on the right.