The Matomo config file is typically located in $WordPressRoot/wp-content/uploads/matomo/config/config.ini.php.

If this is not the case, please open the page Matomo Analytics -> Diagnostics within the WordPress Admin UI and look out for the row that starts with “Config exists”. It should be one of the first rows and you will find the path to the config file on the right. The path ends with config.ini.php.

Config path shown in the Matomo diagnostics page in WordPress