When you use the browserstack.yml file to configure your tests, you use the app parameter to specify your app. To use the same yml file for both Android and iOS apps, you'd then have to manually change the app parameter to switch between your .apk and .ipa files.
To avoid manually changing this, BrowserStack allows you to maintain a yml file for every app that you want to test. This way you can define configurations for testing each app in its own yml file. Refer the multiple config files FAQ in BrowserStack documentation for detailed instructions on how to handle multiple configuration files with the BrowserStack SDK.