Instead of defining append_ for structure

  newtype Structure = Structure String

  append_ :: Structure -> Structure -> Structure
  append_ (Structure c1) (Structure c2) = Structure (c1 <> c2)

  empty_ :: Structure
  empty_ = Structure ""

the <> operator (from Semigroup) can be reused by deriving it via the underlying String type (edit: I see this is suggested later in the tutorial).

  {-# Language DerivingStrategies         #-}
  {-# Language GeneralizedNewtypeDeriving #-}

  newtype Structure = Structure String
    deriving
    newtype (Semigroup, Monoid)

  append_ :: Structure -> Structure -> Structure
  append_ = (<>)

  empty_ :: Structure
  empty_ = mempty

Semigroup and Monoid let us use a lot of standard vocabulary

  concatStructure :: [Structure] -> Structure
  concatStructure list =
    case list of
      [] -> empty_
      x : xs -> x <> concatStructure xs

becomes

  concatStructure :: [Structure] -> Structure
  concatStructure = mconcat

or

  concatStructure = fold

Also in the detour about kinds they are written as * while the ecosystem is moving towards a more uniform Type name.

  {-# Language StandaloneKindSignatures #-}

  import Data.Kind (Type)

  type Tuple :: Type -> Type -> Type
  data Tuple a b = Tuple a b

  type Either :: Type -> Type -> Type
  data Either a b = Left a | Right b