Factory-bot - How to build association and nested

I am new to Factories and I need help for the association and nested attributes....

How do I set an admin user that creates a product? OK
How do I set category to a product? Ok
How do I attach images to a product? OK
How do I set product's sizes (nested attibutes)

user.rb

 has_many :products

product.rb

belongs_to :user
belongs_to :category
has_many :sizes, inverse_of: :product, dependent: :destroy #nested_attributes

size.rb

belongs_to :product

category.rb

has_many :products

factories/users.rb

  FactoryBot.define do
    factory :user do 
        first_name        { Faker::Name.first_name}
        last_name         { Faker::Name.last_name }
        admin             { [false, true].sample }
        sequence(:email)  { |n| "#{n}#{Faker::Internet.email}" }
        birth_date        {"20/10/1997"}
        password          { 'password'}
    end 
  end

factories/categories.rb

FactoryBot.define do
  factory :category do
    title { Faker::Artist.name }
  end
end

factories/sizes.rb

FactoryBot.define do 
    factory :size do 
        size_name {["S", "M", "L", "XL"].sample }
        quantity  { Faker::Number.number(2) }
    end
end

factories/products.rb

FactoryBot.define do
  factory :product do
    title { Faker::Artist.name}
    ref   { Faker::Number.number(10)}
    price { Faker::Number.number(2) }
    color { Faker::Color.color_name }
    brand { Faker::TvShows::BreakingBad }
    description { Faker::Lorem.sentence(3) }
    size
    category
    # how to set an admin ?? 
  end
end

Add associations like this

For product

FactoryBot.define do 
    factory :product do
      user {User.first || association(:user)}
      user {User.first || association(:user, admin: true)}
      # your admin attribute (role: admin or admin: true) whatever you are using for admin
      category {Category.first || association(:category)}
    end
end

Read FactoryBot association hope it will help.