There are many documentation sources relevant to the Wikidata & Wikibase team.
::: tip For user facing Wikibase documentation please see https://www.mediawiki.org/wiki/Wikibase. :::
See: (wmde.github.io/wikidata-wikibase-architecture)
In 2020 an initiative was started to create some architecture documentation for both Wikidata and Wikibase.
This documentation is currently work in progress.
See: (doc.wikimedia.org)
The Wikimedia open source documentation portal contains low level docs for many software components hosted on Gerrit.
Most notably for us this includes the versioned in repo doc sites for the Wikibase.git MediaWiki extensions for PHP code and JS code.
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.
See (www.mediawiki.org)
A higher "user" facing level of documentation exists on the MediaWiki.org site.
This includes a WIP overview of MediaWiki architecture, as well as documentation of individual settings and hooks such as $wgDBname and PageMoveComplete.
Relevant to Wikidata & Wikibase we have a user facing Wikibase documentation page, and documentation pages for individual extensions, such as WikibaseLexeme.
See: (wikitech.wikimedia.org)
Documentation relevant to the Wikimedia deployment of our software primarily lives on WikiTech.
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.
See WiKit Documentation.
Documentation that covers how to use and contribute to WiKit - the Wikidata / Wikibase Design System.
If you are on the team you can also find a variety of resources on the internal Google Drive.