There are many documentation sources relevant to the Wikidata & Wikibase team.
For user facing Wikibase documentation please see https://www.mediawiki.org/wiki/Wikibase (opens new window).
For a WIP high level system overview, checkout the architecture docs. This aims to cover all Wikibase systems and components.
# Wikimedia documentation portal
The Wikimedia open source documentation portal (opens new window) contains low level docs for many software components hosted on Gerrit.
This also includes a variety of story books for UI components, and you can find more by searching for "Wikibase" on the doc site page.
# MediaWiki documentation for "Users"
A higher "user" facing level of documentation exists on the MediaWiki.org site (opens new window).
This includes a WIP overview of MediaWiki architecture (opens new window), as well as documentation of individual settings and hooks such as $wgDBname (opens new window) and PageMoveComplete (opens new window).
Relevant to Wikidata & Wikibase we have a user facing Wikibase documentation page (opens new window), and documentation pages for individual extensions, such as WikibaseLexeme (opens new window).
# Wikimedia deployments
Documentation relevant to the Wikimedia deployment of our software primarily lives on WikiTech (opens new window).
This also coveres services provided by the Wikimedia Foundation such as Cloud VPS, Tool Labs and more.
You can find the Wikidata specific hub on the WMDE/Wikidata page (opens new window).
If you are on the team you can also find a variety of resources on the internal Google Drive.