typespec1-3  Exercise 1: A Recipe Model
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recipe.tsp (compiled with TypeSpec 1.17.0, OpenAPI 3.1.0):

  import "@typespec/http";
  using Http;

  @service(#{ title: "Recipe Service" })
  namespace RecipeService;

  enum Unit { gram, millilitre, piece }

  model Ingredient {
    name: string;
    quantity: decimal;
    unit: Unit;
  }

  model Recipe {
    id: string;
    title: string;
    servings: int32 = 4;
    prepTime: duration;
    ingredients: Ingredient[];
    tags?: string[];
    addedOn: plainDate;
  }

  @route("/recipes") interface Recipes { @get list(): Recipe[]; }

The emitted schemas (the part of the file after 'components:'):

  components:
    schemas:
      Ingredient:
        type: object
        required:
          - name
          - quantity
          - unit
        properties:
          name:
            type: string
          quantity:
            type: number
            format: decimal
          unit:
            $ref: '#/components/schemas/Unit'
      Recipe:
        type: object
        required:
          - id
          - title
          - servings
          - prepTime
          - ingredients
          - addedOn
        properties:
          id:
            type: string
          title:
            type: string
          servings:
            type: integer
            format: int32
            default: 4
          prepTime:
            type: string
            format: duration
          ingredients:
            type: array
            items:
              $ref: '#/components/schemas/Ingredient'
          tags:
            type: array
            items:
              type: string
          addedOn:
            type: string
            format: date
      Unit:
        type: string
        enum:
          - gram
          - millilitre
          - piece

PROPERTY BY PROPERTY (Recipe)
  id          required; type: string
  title       required; type: string
  servings    required AND defaulted (default: 4); integer, format int32. It is in the required list because the property is not marked optional: a default does not make it optional
  prepTime    required; string with format duration (an ISO 8601 duration)
  ingredients required; an array whose items reference the Ingredient schema
  tags        OPTIONAL (the ?); an array of strings; not in the required list
  addedOn     required; string with format date (a plainDate: no time of day)
  (Ingredient) name: string; quantity: decimal, emitted as number with format decimal; unit: a reference to the Unit enum, whose values are gram, millilitre and piece.

WHY THIS WORKS AS AN ANSWER
---------------------------
The model uses one of each kind of thing from the chapter: a default, an optional array, a named enum, a nested model, and three kinds of scalar. Reading the emitted schema line by line, rather than trusting the TypeSpec, is the habit to build. If you want servings to be omittable with a default of 4, change it to servings?: int32 = 4.