This directory can be used to ship various plugins inside an Ansible collection. Each plugin is placed in a folder that is named after the type of plugin it is in. It can also include the module_utils and modules directory that would contain module utils and modules respectively.

Here is an example directory of the majority of plugins currently supported by Ansible:

└── plugins
    ├── action
    ├── become
    ├── cache
    ├── callback
    ├── cliconf
    ├── connection
    ├── filter
    ├── httpapi
    ├── inventory
    ├── lookup
    ├── module_utils
    ├── modules
    ├── netconf
    ├── shell
    ├── strategy
    ├── terminal
    ├── test
    └── vars

A full list of plugin types can be found at Working With Plugins.