Technology

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