20 files declare their use of StreamWrapperManager

config.module in core/modules/config/config.module
Allows site administrators to modify configuration.
CssOptimizer.php in core/lib/Drupal/Core/Asset/CssOptimizer.php
file.api.php in core/lib/Drupal/Core/File/file.api.php
Hooks related to the File management system.
FileCopy.php in core/modules/migrate/src/Plugin/migrate/process/FileCopy.php
FileSystem.php in core/lib/Drupal/Core/File/FileSystem.php
FileUrlGenerator.php in core/lib/Drupal/Core/File/FileUrlGenerator.php
GDToolkit.php in core/modules/system/src/Plugin/ImageToolkit/GDToolkit.php
HtaccessWriter.php in core/lib/Drupal/Core/File/HtaccessWriter.php
image.module in core/modules/image/image.module
Exposes global functionality for creating image styles.
ImageFieldDisplayTest.php in core/modules/image/tests/src/Functional/ImageFieldDisplayTest.php
ImageStyle.php in core/modules/image/src/Entity/ImageStyle.php
ImageStyleCustomStreamWrappersTest.php in core/modules/image/tests/src/Kernel/ImageStyleCustomStreamWrappersTest.php
ImageStyleDownloadController.php in core/modules/image/src/Controller/ImageStyleDownloadController.php
ImageStylesPathAndUrlTest.php in core/modules/image/tests/src/Functional/ImageStylesPathAndUrlTest.php
locale.translation.inc in core/modules/locale/locale.translation.inc
Common API for interface translation.
StreamWrapperManagerTest.php in core/tests/Drupal/KernelTests/Core/StreamWrapper/StreamWrapperManagerTest.php
system.module in core/modules/system/system.module
Configuration system that lets administrators modify the workings of the site.
ThemeSettingsForm.php in core/modules/system/src/Form/ThemeSettingsForm.php
ThemeTest.php in core/modules/system/tests/src/Functional/System/ThemeTest.php
UserPictureTest.php in core/modules/user/tests/src/Functional/UserPictureTest.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.