This gem is intended to simplify your routes file. If you want to split it in multiples files to organize your routes, This is what you need.
gem 'rails_routes_drawer'
Create a simple file under config/routes/my_appended_routes.rb
resources :products do
resources :variants
end
and then in your config/routes.rb, just draw it!!
Rails.application.routes.draw do
...
draw :my_appended_routes
...
end