How can I position some objects in my room using WebXR?

I would like to use WebXR to build an application that displays some objects in the room in some predefined positions. Hence, by using the camera on my phone, I would be able to capture the real world surrounding me, with the superposition of computer-generated content. Basically augmented reality, but with the objects positioned according to the real world room and not to the x-y-z axes of the camera. To start simple, I wanted to build a model of my room and place the objects there, making them visible when using the app. Unfortunately, I can't find anything useful online. Does anyone know a solution to this or have some references to suggest me?

If you are very familiar with WebXR, please let me know because this technology is very new to me (and apparently to the world) and I would like to ask some questions :)

Thank you!