To execute extended deploy scenario run next command from your Magento Cloud project:
php vendor/bin/ece-tools run vendor/magento/ece-tools/scenario/deploy.xml vendor/magento/ece-tools-extend/scenario/extend-deploy.xml
You can extend base scenario with different custom scenarios.
php vendor/bin/ece-tools run "path/to/base/scenarion" "path/to/extended/scenarion" "path/to/extended/scenarion2" "path/to/extended/scenarion3"
Keep in mind that scenarios will be merged in provided order.