Where do modules come from beyond ansible.builtin, and why do module names like community.general.apache2_module have three parts?

Since ansible-core 2.11 modules are organised in content collections; only ansible.builtin is always available. Other collections come with the execution environment or are downloaded from Automation Hub, a private hub or Ansible Galaxy into the project's collections directory. The three-part name is the fully qualified collection name: namespace, collection, module.

A fully qualified collection name (FQCN) reads namespace.collection.module:

The short name apache2_module still works if the collection is installed, but the FQCN says unambiguously which code runs, which matters once two collections ship a module with the same short name. ansible-doc apache2_module shows the full name and the path under ansible_collections/community/general/, so you know where it came from.

Where to get collections:

• ansible.builtin: always there, part of ansible-core.
• Execution environment: the default container image (for example ee-supported-rhel8) already includes more collections.
• Automation Hub (Red Hat, certified content, via console.redhat.com), a private hub, or the community's Ansible Galaxy (galaxy.ansible.com), installed with ansible-galaxy collection install into the project's collections/ directory.
• ansible-navigator collections browses what's available in an interactive UI.

Go deeper:

• Using Ansible collections (official docs) — installing, listing and referencing collections by FQCN.
• Ansible Galaxy — search community collections and roles.
• ansible.builtin collection (official docs) — the collection that is always available.