each_pair (alias each) yields key/value pairs in insertion order; Ruby has guaranteed hash ordering since 1.9. each_key and each_value are convenience iterators when you need only one side of each entry. keys and values return arrays you can chain further map/select on, but for one-shot loops the each-variants avoid building an intermediate array. Hash iteration order makes hashes a natural drop-in for ordered configuration maps; do not assume the same in Python before 3.7 or Java's HashMap.

transform_keys (Ruby 2.5+) and transform_values (Ruby 2.4+) return a new hash with the keys or values run through the block; the OTHER side stays untouched. The &:to_sym form passes a symbol-as-proc. These are the cleanest replacements for the old Hash[h.map { |k, v| [k.to_sym, v] }] idiom that you will still see in older codebases. select and reject on a hash also return a hash (not an array of pairs), which lets you chain hash transformations end-to-end without breaking out of the type.

each_with_object({}) is the cleanest hash-builder: pass the seed object as the argument, mutate it inside the block, and the method returns it for you. The destructuring (name, score) unpacks the key/value pair without a manual pair[0] / pair[1]. The equivalent with inject requires explicitly returning the accumulator at the end of the block, which is easy to forget; prefer each_with_object for hash and array building. For aggregating values, scores.values.sum (or inject(:+)) is shortest; lean on the right tool for the right shape.