I have built some simple PowerApps (not portal) and works fine on Android and iPhone. But I am not sure how to enable MFA on PowerApp. Is there any official document available? I can only find discussion for this topic on other forum. Could you please advise?
Yes, there is and it's quite simple actually.
You can configure Conditional Access with MFA to the specific application for the specific set of users as per your requirement. Read on https://docs.microsoft.com/en-us/azure/active-directory/external-identities/b2b-tutorial-require-mfa#create-a-conditional-access-policy-that-requires-mfa.
Follow the guide, and then on the Cloud Apps, select Microsoft PowerApps. Ensure you've logged out and cleared all browser caches before you test it.
If you are wondering: how to configure MFA on a specific app? Unfortunately no, it will apply to all once it's turned on.
Hope it helps.