Each item in a story carries a type field that names a plugin that the server is expected to provide.
It is possible for a page to travel through the federation beyond servers that know how to render every single item. Should rendering fail the wiki-client will make some attempt to render as plain text and offers a "help" button that suggests a way forward including viewing from the originating server which may be provisioned adequately.
The glossary has some more hints as to where plugins can be found. See Plugin Names
Some plugins are wired into the wiki-client so they need not be "plugged" at all. Browse Paragraph here, and others nearby.
github 
Many more, but not all, plugins are in their own repos in the collectively managed fedwiki organization.
github
A server operator can provision the plugins as they see fit. However, the least effort mechanism is to retrieve published plugins from npm. Npm will list what it knows is available and offer links to source and community.
npm
We recommend plugins be open-sourced and supported by a community before they become an important dependency of content expected to be shared.