I created my own extension for magento 2.3.0.
but all my classes not recognized by magento
Even after running the command bin/magento setup:upgrade
First Run php bin/magento module:status (considering you added the module in app/code directory)
If your module is not showing here then check your etc/module.xml file and registration.php file if its syntax is correct or not.
If its showing then run php bin/magenot module:enable <module_name>
then run php bin/magento setup:upgrade and other commands