So, will it be locked down like the iPhone with everything reviewed by Apple, or more free like the Mac?
The presentation clearly showed that you could use the goggles to display your macbook screen, so in that sense, if you want a screen in 3d space, or maybe even multiple screens in 3d space, you can have a window to whatever your macbook can do.
In the presentation, I believe they showed that they abstracted away the parts of IOS that are common to all systems and built visionOS on top of it, so we are likely to see a locked down IOS like ecosystem.
The flipside is that AR is an unproven technology with no killer apps, so a closed ecosystem and limiting OS api access could stymie innovation that might validate their investment in unproven "future tech."
I think it is plausible that apple might let a steam like service on their platform.