expl.rb

i   = 5
x   = [:Fizz][i%3]   # => :Fizz || nil
#            5%3
#     [:Fizz][2]     # => nil
str = "%sBuzz" % x   # => 'Buzz' || 'FizzBuzz'
#     "%sBuzz" % nil # => 'Buzz'
str = [str][i%5]     # => ^ above str or nil
#             5%5
#     ['Buzz'][0]    # => 'Buzz'
#
#  str   || x   || i # "(Fizz)?Buzz" || "Fizz" || i
# 'Buzz' || nil || 5 

str || x || i        # => 'Buzz'