service1.scala

// UserModel.scala
import zio.{Task, ZIO, ZLayer}

object DB {
  // 1. service
  trait Service {
    def execute(sql: String): Task[Unit]
  }
  
  // 2., 3. omitted here
}

object UserModel {
  // 1. service
  trait Service {
    def insert(u: User): Task[Unit]
  }

  // 2. layer - service implementation
  val live: ZLayer[DB, Nothing, UserModel] = ZLayer.fromService { db =>
    new Service {
      override def insert(u: User): Task[Unit] = 
        db.execute(s"INSERT INTO user VALUES ('${u.name}')")
    }
  }

  // 3. accessor
  def insert(u: User): ZIO[UserModel, Throwable, Unit] = ZIO.accessM(_.get.insert(u))
}

// ---

// 4. type aliases in package.scala
type DB = Has[DB.Service]
type UserModel = Has[UserModel.Service]