Start by installing and enabling modules, remove those which you don't need, now or later.

composer require
  drupal/notifier \
  drupal/notifier_chat_channel \
  drupal/notifier_email_channel \
  drupal/sms \
  drupal/symfony_http_client \
  drupal/sm

drush module:enable
  notifier \
  notifier_chat_channel \
  notifier_email_channel \
  sms \
  symfony_http_client \
  sm

Checklist for a functional Notifier installation:

• Install one or more of Symfony Chat Channel, Symfony Email Channel, or SMS Framework, for whether emails or SMS messages will be sent, respectively.
• Configure Transports — configure the services and transports you'll use, if applicable. Email does not use transports, but it does require a functional Symfony Mailer installation. Fake transports are available for testing purposes.
• Configure Routing (optional) — create a channel router in order to determine which channel/service a notification will route to. Otherwise, a message will be routed to all possible channels. See Custom channel routers.
• Configure Recipients (optional) — out-of-the-box notifications can be sent to users, via email if email channel is configured. Otherwise, you'll need to configure mapping. Entity fields is usually what you want to configure. See Recipients.
• Sending code — create a service or controller to begin programmatically creating SMS messages. See Sending.

Service parameters are used for configuration. See Configuration.