When does sanity give objects a _type and what are the types?

sanity.config.ts

import { visionTool } from "@sanity/vision";
import {
  defineArrayMember,
  defineConfig,
  defineField,
  defineType,
} from "sanity";
import { deskTool } from "sanity/desk";

const theDocument = defineType({
  name: "theDocument",
  title: "TheDocument",
  type: "document",
  fields: [
    defineField({
      name: "inlineFieldObject",
      description: "An Object as a field.",
      type: "object",
      validation: (Rule) => Rule.required(),
      fields: [
        defineField({
          name: "typeIs",
          type: "string",
          initialValue: "",
        }),
      ],
    }),
    defineField({
      name: "aliasedFieldObject",
      description: "An Aliased Object as a field.",
      type: "aliasedObject",
      validation: (Rule) => Rule.required(),
      initialValue: { typeIs: "type" },
    }),
    defineField({
      name: "inlineMemberObjects",
      description: "Objects in an array",
      type: "array",
      validation: (Rule) => Rule.required(),
      of: [
        defineArrayMember({
          type: "object",
          validation: (Rule) => Rule.required(),
          fields: [
            defineField({
              name: "typeIs",
              type: "string",
              initialValue: "",
            }),
          ],
        }),
      ],
    }),
    defineField({
      name: "inlineNamedMemberObjects",
      description: "Objects with names in an array.",
      type: "array",
      validation: (Rule) => Rule.required(),
      of: [
        defineArrayMember({
          name: "inlineNamedMemberObject",
          type: "object",
          validation: (Rule) => Rule.required(),
          fields: [
            defineField({
              name: "typeIs",
              type: "string",
              initialValue: "name",
            }),
          ],
        }),
      ],
    }),
    defineField({
      name: "aliasedMemberObjects",
      description: "Aliased Objects in an array",
      type: "array",
      validation: (Rule) => Rule.required(),
      of: [
        defineArrayMember({
          type: "aliasedObject",
          validation: (Rule) => Rule.required(),
          initialValue: { typeIs: "type" },
        }),
      ],
    }),
    defineField({
      name: "aliasedNamedMemberObjects",
      description: "Aliased Named Objects in an array",
      type: "array",
      validation: (Rule) => Rule.required(),
      of: [
        defineArrayMember({
          name: "namedAliasedObject",
          type: "aliasedObject",
          validation: (Rule) => Rule.required(),
          initialValue: { typeIs: "name" },
        }),
      ],
    }),
  ],
});

const aliasedObject = defineType({
  name: "aliasedObject",
  type: "object",
  fields: [
    defineField({
      name: "typeIs",
      type: "string",
    }),
  ],
});

export default defineConfig({
  name: "default",
  title: "sanity-typed-sandbox",

  projectId: "59t1ed5o",
  dataset: "production",

  plugins: [deskTool(), visionTool()],

  schema: {
    types: [
      // Did you know if you tried to put the `defineType`s in here, typescript gets mad?
      theDocument,
      aliasedObject,
    ],
  },
});

sanityDocumentExample.json

{
  "_createdAt": "2023-06-15T19:27:22Z",
  "_id": "a16452cb-9ae8-4867-a24f-ae0092a251ca",
  "_rev": "HAbzzJwXm1ji6fGARD0F1h",
  "_type": "theDocument",
  "_updatedAt": "2023-06-15T19:27:22Z",
  "aliasedFieldObject": {
    "_type": "aliasedObject",
    "typeIs": "type"
  },
  "aliasedMemberObjects": [
    {
      "_key": "5ea350e950c8",
      "_type": "aliasedObject",
      "typeIs": "type"
    }
  ],
  "aliasedNamedMemberObjects": [
    {
      "_key": "15bccbc2be2c",
      "_type": "namedAliasedObject",
      "typeIs": "name"
    }
  ],
  "inlineFieldObject": {
    "typeIs": ""
  },
  "inlineMemberObjects": [
    {
      "_key": "389b310cf818",
      "typeIs": ""
    }
  ],
  "inlineNamedMemberObjects": [
    {
      "_key": "40db63c5f8dc",
      "_type": "inlineNamedMemberObject",
      "typeIs": "name"
    }
  ]
}

Looks like the order of precedence is:

1. If it's in an array and has a name, that is it's _type.
2. Otherwise, if it's an aliased object, the _type is the aliased object's name.
3. Otherwise, it has no _type.