No problem. My suggestion may not be what you're looking for.
Absolutely, you can do the calculations manually, but it's a lot harder. When you create a parent / child relationship in Unity between game objects, what you're doing is establishing a local coordinate system for the child in the parent's Transform coordinate frame. To get the child's world position, you need to first multiply together the Transforms of all the ancestors from the scene root down to the child. Then decompose the result into a translation, rotation, and scale.
Last time I looked, you couldn't create Transforms outside the scene structure (just by creating game objects). However, you could certainly create a Matrix4x4 (SetTRS) that contains the transform of the tracking space in scene coordinates, and another Matrix4x4 of the local tracker offset. Then multiply them together and decompose the resulting Matrix4x4 into a TRS. I don't think Unity has a method to do that, but you can find one on the internet.
It's a **lot** easier (and efficient) to create a game object for your tracker space in the scene, and then child another game object to represent your (for instance) hand. Then update the hand position (local) with XR tracking data. The Transform.position property of the hand is the position in world coordinates. You don't have to do any math.
↧