...but what is the vendor folder for in my root.
Comes with composer I think and yes I have googled but nothing seems to give me a simple one line definition of vendor folder.

thanks
Martin

I think it's where SilverStripe automatically puts all third-party requirements from the composer.json files that are not SS core ore a module.

So composer is in there because of the requirement in cms, framework and the simple theme:

	"require": {
		"composer/installers": "*",
	},

But I've seen more in there, depending on modules defining their own third-party software requirements