diff --git a/docs/ARCHITECTURE.md b/docs/ARCHITECTURE.md
index ab98c98..a8730b9 100644
--- a/docs/ARCHITECTURE.md
+++ b/docs/ARCHITECTURE.md
@@ -199,24 +199,33 @@ When the user speaks while the AI is talking:
 5. Partial response saved with `[interrupted]` marker in history
 6. AnimationTree: speaking → interrupted (brief surprise) → listening
 
-## Desktop Overlay
+## Platform Rendering
 
-Godot 4 transparent window configuration:
+### Desktop: Transparent Overlay
+
+Miku floats on the desktop — no window chrome, no background. The OS composites the 3D avatar directly over whatever the user is doing.
 
 ```gdscript
-# In project.godot or at runtime:
 DisplayServer.window_set_flag(DisplayServer.WINDOW_FLAG_TRANSPARENT, true)
 DisplayServer.window_set_flag(DisplayServer.WINDOW_FLAG_ALWAYS_ON_TOP, true)
 DisplayServer.window_set_flag(DisplayServer.WINDOW_FLAG_BORDERLESS, true)
-
-# Transparent viewport
 get_viewport().transparent_bg = true
-
-# Click-through on transparent pixels (optional)
-# Handled via input event detection on the character mesh
 ```
 
-The result: the character floats on the desktop with no window chrome, visible above all other windows, with only the character model and minimal UI elements being interactive.
+Desktop-specific features: window drag, zoom, edge snap, system tray integration, keyboard shortcuts, gaze halo overlay.
+
+### Mobile: Fullscreen with Background Modes
+
+Mobile OSes don't support transparent overlay windows — Miku owns the full screen. The background behind the avatar is configurable with four modes:
+
+| Mode | Source | Use case |
+|------|--------|----------|
+| **Camera feed** | Rear/front `CameraFeed` → viewport background | AR-style, companion in the real world. Front camera doubles as face tracking input. |
+| **Rendered environment** | 3D scene (bedroom, park, abstract) | Virtual pet aesthetic, configurable themes |
+| **Camera blur** | Camera feed → Gaussian blur shader | Softer AR look, less visual noise |
+| **Solid/gradient** | Flat color or gradient | Battery-friendly fallback, clean aesthetic |
+
+The background layer renders behind the avatar in the viewport. The avatar, lighting, and UI are identical to desktop — only the background differs. Desktop has transparency as its implicit "background mode" and doesn't use this system.
 
 ## Animation Architecture