Hash may be sorted by the key with the Enumerable#sort method. Internally, in C, it is described as:
return rb_ary_sort(enum_to_a(0, 0, obj));
#sort intended to be used on Hash?
h = {:john => 18, :anne => 24, :beth => 35, :paul => 33}
Sort by key
h.sort.to_h # Output: => {:anne=>24, :beth=>35, :john=>18, :paul=>33}
Sort by value
h.sort_by {|_,v| v}.to_h # Output: => {:john=>18, :anne=>24, :paul=>33, :beth=>35}
Both sort and sort_by methods return arrays, hence the need to convert the result back into a hash with to_h.
Unlike the Array class, which provides its own implementation of the sort method, the Hash class uses the sort method implemented in the Enumerable module.
Array.instance_method(:sort) # Output: => #<UnboundMethod: Array#sort>
Hash.instance_method(:sort) # Output: => #<UnboundMethod: Hash(Enumerable)#sort>
Objects such as numbers and strings, which can be compared (amongst themselves) in terms of being greater or smaller than others, provide the <=> method, also known as the spaceship method. When comparing two objects, <=> returns -1 if the first object is lesser than the second (a < b), 0 in case they are equal (a == b) and 1 when the first object is greater than the second (a > b).
5 <=> 8 # Output: => -1
5 <=> 5 # Output: => 0
8 <=> 5 # Output: => 1
Most comparable or sortable object classes, such as Integer, Float, Time and String, include a mixin called Comparable, which provides the following comparison operators: < (less than), <= (less than or equal), == (equal), > (greater than), >= (greater than or equal). These methods use the spaceship operator under the hood.
Let's find out which classes include the Comparable mixin:
ObjectSpace.each_object(Class).select { |c| c.included_modules.include? Comparable }
# Output: => [Complex, Rational, Time, File::Stat, Bignum, Float, Fixnum, Integer, Numeric, Symbol, String, Gem::Version, IRB::Notifier::NoMsgNotifier, IRB::Notifier::LeveledNotifier]
Comparison operators can be used in objects of all the above classes, as in the following examples.
# String
"a" < "b" # Output: => true
"a" > "b" # Output: => false
# Symbol
:a < :b # Output: => true
:a > :b # Output: => false
# Fixnum (subclass of Integer)
1 < 2 # Output: => true
2 >= 2 # Output: => true
# Float
1.0 < 2.0 # Output: => true
2.0 >= 2.0 # Output: => true
# Time
Time.local(2016, 5, 28) < Time.local(2016, 5, 29) # Output: => true
Most sorting operations use the spaceship (<=>) operator.
Your first question is not clear. If it is to be made clear, it would probably going to be opinion based, which does not have a correct answer.
Regarding your second question, a hash would be converted to an array with to_a, and that guarantees that you get an array of arrays, where each sub-array has a key and the corresponding value of the original hash in this order. You always get:
{:a => :b, :c => :d}.to_a # => [[:a, :b], [:c, :d]]
and not:
{:a => :b, :c => :d}.to_a # => [[:b, :a], [:d, :c]]
Then, sort applies to the result. When an array of arrays is sorted, it is first sorted by the first element of the subarrays, i.e., :a and :c above, which correspond to the keys, not the values, of the original hash. So you can rely on the behavior you are mentioning.