how do I build an immersiveSpace?

To build an immersive space, you can follow these steps as outlined in the session "Dive deep into volumes and immersive spaces":

Create the Immersive Space: You can configure this in the new project dialog in Xcode or add it manually to your project. Initially, the immersive space will be empty.
Transplant Content: When the immersive space is opened, you can move all of the RealityKit content from the volume into the immersive space. This transition should be seamless to provide a delightful experience for the user.
Coordinate Space: Use the named coordinate space introduced in visionOS 1.1 called "immersive space" to precisely specify positions relative to the immersive environment.
Progressive Immersion Style: You can create a progressive immersion style using a new initializer that takes in a custom immersion range and an initial value. This allows you to define the minimum, maximum, and initial values of the progressive effect applied to the scene.

For more detailed guidance, you can refer to the session Dive deep into volumes and immersive spaces starting at 18:40.