If you also distribute your app outside of Google Play or plan to later, you can generate the app signing key you want to use for every app store, and then upload it to Google when you opt in to app signing by Google Play.
If you use Android App Bundles, you need to sign only your app bundle before you upload it to the Play Console, and app signing by Google Play takes care of the rest. However, you can also manually sign your app for upload to Google Play and other app stores.
You generate an upload key/keystore/jks, which you can use to signed the Bundle/APK.
When you setup your release track (Production/Beta/Alpha/Internal test), you can Opt in to app signing by Google Play.
To have Google Play generate an app signing key for you and use it to sign your app, select Continue. The key you use to sign your first release becomes your upload key, and you should use it to sign future releases.
You upload your Bundle/APK using your generated upload key.
NOTE: I have yet to fully test how Google App Signing works and it’s implication. Will update this document once I learned more.