Exercise 1: Implicit Many-to-Many and connectOrCreate — Possible Solution
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schema.prisma (add):

  model Post {
    // ...existing fields
    tags Tag[]
  }

  model Tag {
    id    Int    @id @default(autoincrement())
    name  String @unique
    posts Post[]
  }

  $ npx prisma migrate dev --name add-tags
  $ npx prisma generate

Make sure the "prisma" tag already exists:

  await prisma.tag.create({ data: { name: "prisma" } });

One create call:

  const post = await prisma.post.create({
    data: {
      title: "Tagging posts",
      slug:  "tagging-posts",
      author: { connect: { email: "alan@example.com" } },
      tags: {
        connectOrCreate: [
          { where: { name: "prisma" },     create: { name: "prisma" } },
          { where: { name: "typescript" }, create: { name: "typescript" } },
        ],
      },
    },
    include: { tags: true },
  });
  console.log(post.tags.map((t) => t.name));   // [ 'prisma', 'typescript' ]

"prisma" is linked (connected), and "typescript" is created and linked.

In Prisma Studio: the Tag table has both tags, and the relation is
shown on each Post and Tag. The underlying join table, _PostToTag,
holds two rows linking the post's id to the two tag ids.

WHY THIS WORKS AS AN ANSWER
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connectOrCreate handles "existing" and "new" tags in one call, and the
author is linked with connect rather than a foreign key, which is the
style nested writes need. The whole create runs as a single
transaction.