Conventional notations
Application components, including modules, themes, and language packages technically can be located anywhere under the application's root directory. This refers to both default and custom components.
The following relative paths are used for modules and themes:
<theme_dir>:
Theme directory. Usually used when talking about custom themes, or any theme in general.
For out-of-the-box frontend themes, usually one of the following:
app/design/frontend/<Vendor>/<theme>vendor/magento/theme-frontend-<theme><module_dir>
Module directory. When talking about a particular module, also notation similar to the following is used: <Magento_Checkout_module_dir>
For modules, the absolute path is usually one of the following:
app/code/<Vendor>/<Module>vendor/magento/module-<module>-<name>
