only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstaller.php
@@ -161,13 +161,17 @@ public function installDefaultConfig($type, $name) {
       if (is_dir($optional_install_path)) {
         // Install any optional config the module provides.
         $storage = new FileStorage($optional_install_path, StorageInterface::DEFAULT_COLLECTION);
-        $this->installOptionalConfig($storage, '');
+        foreach ($collection_info->getCollectionNames() as $collection) {
+          $this->installOptionalConfig($storage, '', $collection);
+        }
       }
       // Install any optional configuration entities whose dependencies can now
       // be met. This searches all the installed modules config/optional
       // directories.
       $storage = new ExtensionInstallStorage($this->getActiveStorages(StorageInterface::DEFAULT_COLLECTION), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, StorageInterface::DEFAULT_COLLECTION, FALSE, $this->installProfile);
-      $this->installOptionalConfig($storage, [$type => $name]);
+      foreach ($collection_info->getCollectionNames() as $collection) {
+        $this->installOptionalConfig($storage, [$type => $name], $collection);
+      }
     }
 
     // Reset all the static caches and list caches.
@@ -177,22 +181,26 @@ public function installDefaultConfig($type, $name) {
   /**
    * {@inheritdoc}
    */
-  public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = []) {
+  public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = [], $collection = StorageInterface::DEFAULT_COLLECTION) {
     $profile = $this->drupalGetProfile();
     $enabled_extensions = $this->getEnabledExtensions();
     $existing_config = $this->getActiveStorages()->listAll();
 
+    if (!empty($storage) && $storage->getCollectionName() != $collection) {
+      $storage = $storage->createCollection($collection);
+    }
+
     // Create the storages to read configuration from.
     if (!$storage) {
       // Search the install profile's optional configuration too.
-      $storage = new ExtensionInstallStorage($this->getActiveStorages(StorageInterface::DEFAULT_COLLECTION), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, StorageInterface::DEFAULT_COLLECTION, TRUE, $this->installProfile);
+      $storage = new ExtensionInstallStorage($this->getActiveStorages($collection), InstallStorage::CONFIG_OPTIONAL_DIRECTORY, $collection, TRUE, $this->installProfile);
       // The extension install storage ensures that overrides are used.
       $profile_storage = NULL;
     }
     elseif (!empty($profile)) {
       // Creates a profile storage to search for overrides.
       $profile_install_path = $this->extensionPathResolver->getPath('module', $profile) . '/' . InstallStorage::CONFIG_OPTIONAL_DIRECTORY;
-      $profile_storage = new FileStorage($profile_install_path, StorageInterface::DEFAULT_COLLECTION);
+      $profile_storage = new FileStorage($profile_install_path, $collection);
     }
     else {
       // Profile has not been set yet. For example during the first steps of the
@@ -211,7 +219,10 @@ public function installOptionalConfig(StorageInterface $storage = NULL, $depende
 
     // Filter the list of configuration to only include configuration that
     // should be created.
-    $list = array_filter($list, function ($config_name) use ($existing_config) {
+    $list = array_filter($list, function ($config_name) use ($existing_config, $collection) {
+      if ($collection != StorageInterface::DEFAULT_COLLECTION) {
+        return TRUE;
+      }
       // Only list configuration that:
       // - does not already exist
       // - is a configuration entity (this also excludes config that has an
@@ -257,7 +268,7 @@ public function installOptionalConfig(StorageInterface $storage = NULL, $depende
 
     // Create the optional configuration if there is any left after filtering.
     if (!empty($config_to_create)) {
-      $this->createConfiguration(StorageInterface::DEFAULT_COLLECTION, $config_to_create);
+      $this->createConfiguration($collection, $config_to_create);
     }
   }
 
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/Config/ConfigInstallerInterface.php
+++ b/core/lib/Drupal/Core/Config/ConfigInstallerInterface.php
@@ -50,8 +50,10 @@ public function installDefaultConfig($type, $name);
    *   this dependency. The format is dependency type as the key ('module',
    *   'theme', or 'config') and the dependency name as the value
    *   ('book', 'olivero', 'views.view.frontpage').
+   * @param string $collection
+   *   (optional) The configuration collection.
    */
-  public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = []);
+  public function installOptionalConfig(StorageInterface $storage = NULL, $dependency = [], $collection = StorageInterface::DEFAULT_COLLECTION);
 
   /**
    * Installs all default configuration in the specified collection.
only in patch2:
unchanged:
--- a/core/lib/Drupal/Core/ProxyClass/Config/ConfigInstaller.php
+++ b/core/lib/Drupal/Core/ProxyClass/Config/ConfigInstaller.php
@@ -79,9 +79,9 @@ public function installDefaultConfig($type, $name)
          * {@inheritdoc}
          */
         public function installOptionalConfig(?\Drupal\Core\Config\StorageInterface $storage = NULL, $dependency = array (
-        ))
+        ), $collection = '')
         {
-            return $this->lazyLoadItself()->installOptionalConfig($storage, $dependency);
+            return $this->lazyLoadItself()->installOptionalConfig($storage, $dependency, $collection);
         }
 
         /**