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Docker merges layers using **union filesystem mechanisms (OverlayFS, AUFS, and others)**. Each layer is a separate read-only piece of the file system, and when an image is built these layers are **logically "stacked" on top of each other**, forming a single visible root-fs for the container.

### **How this works step by step**

1. **Each Dockerfile instruction creates a layer**
`FROM`, `RUN`, `COPY`, and others -> a new layer (read-only).
2. **Layers are not merged physically**
Docker does not copy the contents of each layer into the next one. Instead, it uses a union-mount that makes them **a single directory tree**.
3. **OverlayFS combines them through the** `lowerdir/upperdir` **mechanisms**
The basic scheme:

| Component | What it is |
  |---|---|
  | `lowerdir` | the image layers (read-only, one on top of another) |
  | `upperdir` | the layer with changes (while the container is running) |
  | `merged` | the final, mounted file system |
4. **When a file changes - CoW (copy-on-write)**
If the container changes a file from a lower layer, Docker **copies it into the upper layer** and the edit happens there, without touching the base layers.

### **What this gives you**

| Advantage | Because of what |
|---|---|
| Space savings | layers are reused by different images |
| Fast builds | layer cache, previously created layers are not rebuilt |
| Immutability | lower layers are read-only and do not get corrupted |

### **The main idea for middle level**

> Docker does not "glue" layers into one physical layer. It **mounts them on top of each other using OverlayFS**, and changes in the container happen only in the top writable layer, using CoW.

If you want, I can add the difference **between OverlayFS and AUFS** or break down this same process at the level of the system directories (`/var/lib/docker/overlay2`).

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