> OpenJDK is an open distribution and preinstalled on all major operating systems

My understanding is that it is preinstalled on chrome os and android, but is not preinstalled for any major linux distribution, is a separate third-party download for windows and Mac, and is unavailable for iOS.

On the contrary, Linux distributions ship only OpenJDK.

They ship it, but it doesn't come pre-installed.

I don't see how this is an argument though, there is no Java in a standard desktop/server installation, but it's trivial to install if needed.

Since when OpenJDK is preinstalled on Windows?

... and iOS? And I’ve only done upgrades, but I don’t believe any JVM is preinstalled on MacOS.

Apple used to have its own JVM implementation.

As for iOS, RoboVM, Codename ONE, GluonVM are three possible AOT compilers to native code.

Apple's JVM was MRJ, the Macintosh Runtime for Java. It became obsolete around 2004, during the transition to Mac OS X.

Last time I used MacOS, it prompted me to install Java and then it did the install. I was in the Terminal, I am not sure what the interaction would look like if I had double-clicked a JAR file.

I believe Apple donated their customizations (Swing L&F, launcher code, etc.) to OpenJDK.

This is Apple's old build of Java 6, and is currently downloaded as needed for backward compatibility. Anything newer than that the third party is expected to bundle into their application.