iOS has two built-in messaging apps. Like all phones, they have SMS built in, and hardly anyone uses it for anything except SMS 2FA codes.

And then they have iMessage, aka blue bubbles, which are kinda like Signal or Whatsapp or Telegram. Everyone in Europe uses whatsapp, and a lot of people in the US use iMessage. If you don't use whatsapp in europe, you'll have a rough time communicating with some social groups, and the same thing for iMessage in the US.

However, unlike every other messenger app I can think of, iMessage isn't cross platform.

Also unlike every other messenger I can think of, it comes installed by default and for some reason uses the same app as the SMS app, and also claims encryption but randomly switches to SMS and breaks encryption making it obviously the least secure of all the apps (and also backs up your keys to iCloud in a way apple can access them by default, neither here nor there).

Blue bubbles are when iMessage is acting as the iMessage app, and has encryption and can use features like sending high resolution photos, location, invites, and a bunch of other apple-specific features.

Green bubbles are when the iMessage app has converted itself into the SMS and RCS app, and has a reduced feature set, like being unable to remove people from group chats.

It's frankly a quite confusing decision to have two quite different apps built into the same app and indicate which feature-set is active based on the color of a UI element. I think everyone would prefer if apple split it into the 'Messages' app (SMS + RCS) and an optional 'iMessage' app which doesn't come installed by default, but you can download on the app store from Apple. I'm frankly surprised the EU hasn't forced apple to show a prompt for "default messenger app" on startup with the options being "Whatsapp", "iMessage", etc etc, like they do for default browser.