Export the model from Blender as a GLB, get the file onto the headset, and open it in Model Explorer at 0xr.space. It appears in your room at the size you modelled it, and you can walk around it, cut through it with a section plane and pull its parts apart. There is no game engine project to build and the file is never uploaded.
Blender works in metres by default and glTF is defined in metres, so a model built to real dimensions arrives at real size. That is the point of the exercise: a chair that looked right on a monitor is sometimes 20 cm too deep once it is standing next to you.
No. The headset browser supports WebXR, so a web page can show the model in the room. Nothing is built, signed or sideloaded.
Almost always unapplied scale or a unit mismatch in the scene. Apply scale before exporting and keep the scene in metres. The viewer shows the bounding box dimensions, so you can see what size the file really says.
Principled BSDF materials export to glTF's PBR model and display as such, and animations that glTF can carry play back. Procedural node textures do not export from Blender at all, so bake them to images first.
Not in Model Explorer: you export a file and open it. If a wireless push from Blender matters more to you than inspection tools, the Model Viewer app on SideQuest ships a Blender add-on for that.