The Ruby on Rails just like its name, I think it's very easier to understand the web application building.
https://guides.rubyonrails.org/getting_started.html#say-hello-rails
First step , using command line to generate the three architecture.

$ rails generate controller Welcome index

The response blew this in command line :

create  app/controllers/welcome_controller.rb
 route  get 'welcome/index'
invoke  erb
create    app/views/welcome
create    app/views/welcome/index.html.erb
invoke  test_unit
create    test/controllers/welcome_controller_test.rb
invoke  helper
create    app/helpers/welcome_helper.rb
invoke    test_unit
invoke  assets
invoke    scss
create      app/assets/stylesheets/welcome.scss
 

Then how to change the content in the website ?
Check the route of app/views/welcome/index.html.erb and change somethings
And then if I want using this new page change the domain page that I will add

Rails.application.routes.draw do
  get 'hello/hello'
  # For details on the DSL available within this file, see https://guides.rubyonrails.org/routing.html
  # change the web domain page to hello#hello use the keyword root
  root 'hello#hello'
end
 

root 'some#some' in this file : projectName/config/routes.rb
So,You just refresh the browser and the new page will appeared

And also,run the command in command line

$ rails routes

You will see the routes in this project

                               Prefix Verb   URI Pattern                                                                              Controller#Action
                          hello_hello GET    /hello/hello(.:format)                                                                   hello#hello
                                 root GET    /                                                                                        hello#hello
        rails_postmark_inbound_emails POST   /rails/action_mailbox/postmark/inbound_emails(.:format)                                  action_mailbox/ingresses/postmark/inbound_emails#create
           rails_relay_inbound_emails POST   /rails/action_mailbox/relay/inbound_emails(.:format)                                     action_mailbox/ingresses/relay/inbound_emails#create
        rails_sendgrid_inbound_emails POST   /rails/action_mailbox/sendgrid/inbound_emails(.:format)                                  action_mailbox/ingresses/sendgrid/inbound_emails#create
  rails_mandrill_inbound_health_check GET    /rails/action_mailbox/mandrill/inbound_emails(.:format)                                  action_mailbox/ingresses/mandrill/inbound_emails#health_check
        rails_mandrill_inbound_emails POST   /rails/action_mailbox/mandrill/inbound_emails(.:format)                                  action_mailbox/ingresses/mandrill/inbound_emails#create
         rails_mailgun_inbound_emails POST   /rails/action_mailbox/mailgun/inbound_emails/mime(.:format)                              action_mailbox/ingresses/mailgun/inbound_emails#create
       rails_conductor_inbound_emails GET    /rails/conductor/action_mailbox/inbound_emails(.:format)                                 rails/conductor/action_mailbox/inbound_emails#index
                                      POST   /rails/conductor/action_mailbox/inbound_emails(.:format)                                 rails/conductor/action_mailbox/inbound_emails#create
    new_rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/new(.:format)                             rails/conductor/action_mailbox/inbound_emails#new
   edit_rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/:id/edit(.:format)                        rails/conductor/action_mailbox/inbound_emails#edit
        rails_conductor_inbound_email GET    /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                             rails/conductor/action_mailbox/inbound_emails#show
                                      PATCH  /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                             rails/conductor/action_mailbox/inbound_emails#update
                                      PUT    /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                             rails/conductor/action_mailbox/inbound_emails#update
                                      DELETE /rails/conductor/action_mailbox/inbound_emails/:id(.:format)                             rails/conductor/action_mailbox/inbound_emails#destroy
rails_conductor_inbound_email_reroute POST   /rails/conductor/action_mailbox/:inbound_email_id/reroute(.:format)                      rails/conductor/action_mailbox/reroutes#create
                   rails_service_blob GET    /rails/active_storage/blobs/:signed_id/*filename(.:format)                               active_storage/blobs#show
            rails_blob_representation GET    /rails/active_storage/representations/:signed_blob_id/:variation_key/*filename(.:format) active_storage/representations#show
                   rails_disk_service GET    /rails/active_storage/disk/:encoded_key/*filename(.:format)                              active_storage/disk#show
            update_rails_disk_service PUT    /rails/active_storage/disk/:encoded_token(.:format)                                      active_storage/disk#update
                 rails_direct_uploads POST   /rails/active_storage/direct_uploads(.:format)                                           active_storage/direct_uploads#create

If you run rails routes, you'll see that it has defined routes for all the standard RESTful actions. The meaning of the prefix column (and other columns) will be seen later, but for now notice that Rails has inferred the singular form article and makes meaningful use of the distinction.