gistfile1.rb

class ActionDispatch::Routing::Mapper
  def draw(routes_name)
    instance_eval(File.read(Rails.root.join("config/routes/#{routes_name}.rb")))
  end
end

BCX::Application.routes.draw do
  draw :api
  draw :account
  draw :session
  draw :people_and_groups
  draw :projects
  draw :calendars
  draw :legacy_slugs
  draw :ensembles_and_buckets
  draw :globals
  draw :monitoring
  draw :mail_attachments
  draw :message_preview
  draw :misc

  root to: 'projects#index'
end

Guys, it's very outdated. You now can just require other routes files without monkey patching, they just have to repeat the Rails.application.routes.draw do:
config/routes.rb:

Rails.application.routes.draw do
  load Rails.root.join("config/routes/dev.rb")
  #...
end

config/routes/dev.rb:

Rails.application.routes.draw do
  scope "dev" do
    # ...
  end
end

No dynamic reloading tho, probably @sharipov-ru's reloader still works.

Anyone found a nice solution on how to seperate route files and still make them auto reloading during development?

Edit: Going now with a combination of the mentioned solutions...

# config/environments/development.rb
class RoutesReloader
  def initialize(app)
    @app = app

    @routes_reloader = ActiveSupport::FileUpdateChecker.new([], 'config/routes' => 'rb') do
      Rails.application.reload_routes!
    end
  end

  def call(env)
    @routes_reloader.execute_if_updated

    @app.call(env)
  end
end

Rails.application.routes.draw do
  ...
  config.middleware.use RoutesReloader
end

# config/routes.rb
Rails.application.routes.draw do
  routes = [:admin_routes, :admin_routes_old, :public_routes, :api_routes]
  routes.each{ |route_file| load Rails.root.join("config", "routes", "#{route_file}.rb") }
end

How can I use concern with this conven?
https://guides.rubyonrails.org/routing.html#routing-concerns

concern :commentable do
  resources :comments
end
resources :messages, concerns: :commentable

can I do this?

draw :api, concerns: :commentable

or there another way?