So when the server has the key, how can you say it's encrypted?

I mean in context of signal we don't just talk about some form of transport encryption but e2e

It's various levels of encryption that are acceptable depending on your risk level.

I'm mainly concerned about SMS spoofing and mass surveillance. iMessage protects against that. The only way the government can read your messages is by serving Apple with a warrant to obtain your iCloud backup.

If I had a lower risk tolerance, I would disable iCloud backups to improve my security.

I mean you followed Snowden right?

I would argue that if you are relevant enough apple and sms are both equally easy to get for them.

If we talk here about signal, I don't think the alternative should be iMessage but telegram etc.

Anecdotal evidence, but I've never had someone ask me to join Telegram.

In terms of encrypted messaging, the popularity in my friend group is:

1. iMessage

2. Signal

3. WhatsApp (only for talking to non-US people)

4. Matrix

I don't think people are only concerned about the government, but rather the corporations that own your data (via storing the encryption key to use whenever they want, to look through whatever they want)

In the end signal and imessage (with backups off) provide roughly the same claimed guarantee here (on a technical level at least). They both promise not to secretly inject additional client keys into your account, (to at least obtain a forward-operating 'wire tap'), but the software you're running is all opaque and could certainly do so if they wanted (or were compelled) to.

fwiw, iMessage is actually E2E encrypted (without Apple storing the key) if you either don't have iCloud backup enabled OR don't enable "Message in the Cloud".