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. Starting with VMware Cloud Foundation 5.1, you can use the SDDC Manager UI to create a VI workload domain with advanced switch configurations.
Alternatively, you can use the Workflow Optimization script to create a VI workload domain. See Create a VxRail VI Workload Domain Using the Workflow Optimization Script. The Workflow Optimization script supports using a JSON file for cluster configuration.