It’s recommended to use Varjo OpenXR plugin to get access for all the supported features. Varjo OpenXR plugin automatically enables built-in OpenXR plugins. If you want to use OpenXR without Varjo-specific features, you can do enable OpenXR following these steps.
Enabling OpenXR in Unreal without Varjo-specific features
Open an existing Unreal project or create a new one.
Enable OpenXR plugin.
Continue following the steps in Getting Started with Unreal.