Update: In create-qna.dto.ts, I added
@ApiProperty()
@IsNumber()
@type(() => CreateUserDto)
user: CreateUserDto;

but then error:

src/qnas/qnas.service.ts:13:38 - error TS2345: Argument of type 'CreateqnaDto' is not assignable to parameter of type 'Omit<qna, "id" | "createdAt" | "updatedAt">'.
Types of property 'user' are incompatible.
Type 'CreateUserDto' is missing the following properties from type 'User': id, createdAt, updatedAt, deletedAt

13 return this.qnaRepository.create(createqnaDto);
~~~~~~~~~~~~

[8:07:08 PM] Found 1 error. Watching for file changes.

Your coding conventions are so hard to understand.
I fixed it by following:

My dto
export class CreateqnaDto {
@ApiProperty()
@IsString()
body: string;

@ApiProperty()
@IsString()
title: string;

@ApiProperty({
description: 'The ID of the user who created the QnA',
example: 1,
})
@isnotempty()
@IsNumber()
userId: number;
}

And in service
async create(createQnaDto: CreateqnaDto) {
// Resolve the user from the userId
const user = await this.userRepository.findById(createQnaDto.userId);
if (!user) {
throw new Error('User not found');
}

const clonedObject = {
  ...createQnaDto,
  user: user,
};
return this.qnaRepository.create(clonedObject);

}