TL/DR:
- 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
Signal represent. Metadata is pretty important. Shortly after snowden, I saw a presentation of a guy, making the claim (paraphrasing) metadata writes a story about you, that isn’t necessarily true. Describing tracking a persons movement through their credit card on public transport. Through bad timing, perhaps their activity coincides with some activist activity. That sort of thing.
People have gone to jail with little more than metadata convicting them. Metadata is everything.