According to suggestion by google, it says
Signing Considarations:
You should sign all of your APKs with the same certificate throughout
the expected lifespan of your apps. There are several reasons why you
should do so
App upgrade: When the system is installing an update to an app, it compares the certificate(s) in the new version with those in the
existing version. The system allows the update if the certificates
match. If you sign the new version with a different certificate, you
must assign a different package name to the app—in this case, the user
installs the new version as a completely new app.
App modularity: Android allows APKs signed by the same certificate to run in the same process, if the apps so request, so that the system
treats them as a single app. In this way you can deploy your app in
modules, and users can update each of the modules independently.
Code/data sharing through permissions: Android provides signature-based permissions enforcement, so that an app can expose
functionality to another app that is signed with a specified
certificate. By signing multiple APKs with the same certificate and
using signature-based permissions checks, your apps can share code and
data in a secure manner.
check https://developer.android.com/studio/publish/app-signing.html#strategies
read this answer to understant more on keystore, certificates and alias