I had upload my app bundle to play console with my signing key(key store file) Google uses it as an upload key certificate and generated a public signing certificate.
When a user downloaded my app from play store it will download with Google generated public signing certificate. Now I need to upload my app to other apps stores. If I use my upload certificate key then it will mismatch with app downloaded from play store.
So how to sign my APP with that Google generated public signing certificate?
If you haven't provided Google with your own signing key when enrolling in Play Signing, then you can't sign the APK yourself.
If you upload an App Bundle to the Play Console, you can download the universal APK from the "Bundle Explorer" page in the Play Console. This APK will be signed with the signing key (i.e. the one on installed on users' devices).
If you upload an APK, then you can simply download the APK signed with the signing key in the Play Console (also in "Bundle Explorer").
It does seem a bit weird though that your users would download from Play but then upgrade from another store, so worth thinking if it's worth the extra hassle to share the signing key across stores.