You can create a VxRail VI workload domain using the SDDC Manager UI or using the Workflow Optimization script.

When you use the product UI, you complete the steps in the SDDC Manager UI. This following documentation describes the process of creating a workload domain using the product UI.

Alternatively, you can use the Workflow Optimization script to perform all of the steps to create a VI workload domain in one place. See Create a VxRail VI Workload Domain Using the Workflow Optimization Script.
Note: The SDDC Manager UI supports creating VxRail VI workload domains with only predefined NIC profiles. If you want to use an advanced NIC profile or separate vDS as overlay traffic, then you need to use the Workflow Optimization script.