Topic on Manual talk:Extension registration

e.g. with $wgMyExtVarname = 'site-specific value'; in LocalSettings.php.

There is also no documentation on the PHP code corresponding to the config section for version 2. The reader is left to experiment.

(I assume you mean 'extension.json'.)

You change extensions' config values the same way in version 2 of the extension.json schema, e.g. with $wgMyExtVarname = 'site-specific value'; in LocalSettings.php.