Messages is the same on OSX and iOS.

It's not deeply integrated into the iOS by any normal definition. It's just shipped together.

fwiw it hasn't been called "OSX" for awhile now

The Messages app in macOS is less capable than the Messages app in iOS. It cannot even edit sent messages.

It can, by right clicking the desired message to edit. This is in macOS Sonoma, and I believe was a part of Ventura as well.

Oh interesting, I have a 2015 MacBook Air. Wonder if the feature is not available on whatever macOS version I have.

It’s a Ventura and later feature and your MacBook Air probably topped out around Monterey or earlier. 2016 MacBooks Pro also didn’t make the cut for Ventura.

Messages has a bunch of special privileges on iOS, which is why they had to add the whole Blastdoor protection framework and why it's such a juicy target for sandbox escape exploits.

Nope. It just happens to be on everyone’s device and usually enabled

Yes, and when it's enabled it has more privileges than most other apps, doesn't it? But yeah you can still remove the app.

Btw, maybe related, on iOS I have "app privacy report" enabled, to show me a list of apps and the recent entitlements they used. Every Apple app, even those that don't need access to them, is shown as having recently accessed my Contacts. I find this weird. Anyone know why they do that? e.g. I've never even used the Health app and yet it's accessing my Contacts for some reason.

It’s basically the same as any other app, there are some special permissions it has to integrate with the OS a bit better but nothing too interesting. Not sure what’s going on with Contacts but it might be a bug?