For those just looking for a second screen with their iPad or MacBook, Universal Display does exactly this, given you’re already in the Apple ecosystem.
Second CPU would be nice, too.
Second CPU would be nice, too.
The built-in MacOS feature that does this called continuity… but I know it was renamed once (used to be sidecar). Maybe you’re referring to an old name? Or is it a product?
Edit: I looked it up and it appears that the app is continuity and “Sidecar” and “Universal Control” are features. IDT those names show up in the OS UX though.