Other then using the same package manager there is very little overlap between the node ecosystem and react's, so I'm not sure what you are referring to
javascript engine provided by the OS, in iOS it's JavascriptCore, in UWP it's the chakra engine.
Not sure what it is in Ubuntu, probably v8 packaged with the executable.
In either case, it's not using node or v8/chromium .
v8 is just a javascript engine, node.js is a platform built on v8.
Node.js adds various standard native modules - mostly various IO (file, tcp etc...), a way to load other modules, access to the underlying OS (environment variables etc...).
running v8 alone is akin to running the browser without html or the dom api, you basically can compile and run javascript functions, but can't do much with it.
