Extension Attributes allow developers to “extend” existing API Data interfaces by defining additional attributes in an XML-based configuration. The phpstan-magento extension parses these XML definitions and generates the respective interfaces so that PHPStan can analyze these interfaces.
Out-of-the-box phpstan-magento will parse all "extension_attributes.xml" files recursively found in the current working directory (the directory that contains the phpstan.neon configuration file). You can change this behavior by defining a "magentoRoot" parameter in your phpstan.neon file that points to the directory that should be scanned instead:
Disabled Magento modules are not yet taken into account. phpstan-magento will parse all extension_attributes.xml files found in all "etc" directories, regardless if the module is enabled or not.