TL/DR:

WhatsApp

  • Yes, the content of your conversations is fully encrypted
  • Meta (aka Facebook) still has access to the metadata: who you chat with, how long you chat, your phone’s battery level, stuff like that
  • You could use Signal to have even better privacy, but your contacts might not use Signal

Apple

  • Focusing on health information specifically (their recent marketing campaign), it’s true that Apple end-to-end encrypts your health data
  • However, other apps may not
  • Third party apps can request (from you) access to your health data
  • Be skeptical of them