Introduction to Composer | Magento 2 Developer Documentation

We use Composer to manage Magento components and their dependencies. Using Composer to get the Magento software metapackage provides the following advantages:

Reuse third-party libraries without bundling them with source code
Reduce extension conflicts and compatibility issues by using a component-based architecture with robust dependency management
Adhere to PHP-Framework Interoperability Group (FIG) standards
Repackage Magento Open Source with other components
Use the Magento software in a production environment

You must create a Composer project from our metapackage if you want to use the Magento Web Setup Wizard to upgrade the Magento software and third-party extensions.

The Web Setup Wizard is being deprecated in Magento 2.3.6 and will be removed in Magento 2.4.0. After it is removed, you must use the command line to install or upgrade Magento.

Links to Magento code

The key to developing any Magento component is its composer.json, which specifies version and dependency information for a component, among other things. You can look at Magento 2 code, such as:

Metapackage, look at <magento_root>/composer.json.
Customer module
Luma theme
en_us language package

For more information

For more information about Composer, see the Composer documentation.