meza comes pre-built with many extensions, but if you need additional extensions you can add them to any configuration file. The recommended method for adding extensions to all wikis is to use your "postLocalSettings_allWikis.php" file in /opt/meza/config/local. This file may not already exist, but if you add it meza will automatically start using it.
An example file is located at /opt/meza/config/template/more-extensions.php. This shows a method to load extensions for all wikis or just for select wikis.
After you've moved this file into postLocalSettings_allWikis.php, or included it from postLocalSettings_allWikis.php, you need to perform the installation. To do that run:
sudo WIKI=<wiki-id> php /opt/meza/htdocs/mediawiki/extensions/ExtensionLoader/updateExtensions.php
Replace <wiki-id> with any wiki ID. If the extensions you are installing require database updates (e.g. if their install instructions tell you to run update.php) then you will need to run update.php for all wikis. To do that, run the following:
sudo WIKI=<wiki-id> php /opt/meza/htdocs/mediawiki/maintenance/update.php
Do the command above for all wiki IDs.