How do I extend the schema of an existing config_object? Module A defines the following schema in module_a.schema.yml.
module_a.settings.api:
  type: config_object
  label: API settings
  mapping:
    store:
      type: string
      label: Store code
    credentials:
      type: mapping
      label: Credentials
      mapping:
        username:
          type: string
          label: Username
        password:
          type: string
          label: Password

Module B extends the Module A configuration form and defines additional schema properties in module_b.schema.yml.
module_a.settings.api:
  mapping:
    additionl_one:
      type: mapping
      label: Additional one
      mapping:
        additionl_one_sub:
          type: label
          label: Additional one sub

The definition from module_b.schema.yml isn't picked up by Drupal. I'm checking with config_inspector and proving it by the fact that the new properties can't be translated.
Available articles/tutorials out there are doing the same thing but seems like this is either outdated or never worked:

https://www.drupal.org/forum/support/post-installation/2019-02-06/add-a-new-custom-field-to-site-infomation-form-in-drupal8

http://www.jaypan.com/tutorial/drupal-8-extending-core-configuration-extending-core-forms-and-overriding-core-routes#comment-33507
Use hook_config_schema_info_alter().
function module_b_config_schema_info_alter(&amp;$definitions) {
  $definitions['module_a.settings.api']['mapping']['additionl_one'] = [...];
}