Apps (such as Signal) that care about end-to-end encryption do their own key management. So, Apple / Google servers only ever see ciphertext, and don't have access to the key material that's used for the encryption.
Afaik, e2e messengers don't include ciphertext with push notifications. It's an empty push to wake the client. Then the client contacts the origin to fetch the ciphertext.
A Signal developer 12 days ago said Our FCM and APN notifications are empty and just tell the app to wake up, fetch encrypted messages, decrypt them, and then generate the notification ourselves locally.[1]