A Site Design is not indented to create a site, is intended to extend an existing site so you first need to create your site using the UI or PS:
1) Today you can only create modern team sites and communication sites from the SPO UI so if Groups creation is enabled, when you create a modern team site a group is also created. Microsoft does not provide a UI to create modern team sites not connected to Groups...that's only possible if Groups creation is disabled
2) You can create a modern team site not connected to a Group using standard SPO cmdelts or PnP cmdlets...you have to use STS#3 site template