Nested Data Structures Shallow Copy Question

part1.rb

arr1 = ["a", "b", "c"]
arr2 = arr1.dup
p arr1.object_id
p arr2.object_id
p arr1.object_id == arr2.object_id # false
p arr1[0].object_id
p arr2[0].object_id
p arr1[0].object_id == arr2[0].object_id # true
arr2.map! do |char|
  char.upcase
end

p arr1 # => ["a", "b", "c"] 
p arr2 # => ["A", "B", "C"]

p arr1.object_id
p arr2.object_id
p arr1.object_id == arr2.object_id # false
p arr1[0].object_id
p arr2[0].object_id
p arr1[0].object_id == arr2[0].object_id # false

part2.rb

arr1 = ["a", "b", "c"]
arr2 = arr1.dup
p arr1.object_id
p arr2.object_id
p arr1.object_id == arr2.object_id # false
p arr1[0].object_id
p arr2[0].object_id
p arr1[0].object_id == arr2[0].object_id # true
arr2.map do |char|
  char.upcase!
end

p arr1 # => ["A", "B", "C"] 
p arr2 # => ["A", "B", "C"]

p arr1.object_id
p arr2.object_id
p arr1.object_id == arr2.object_id # false
p arr1[0].object_id
p arr2[0].object_id
p arr1[0].object_id == arr2[0].object_id # true