Unity UI, input and animation

1. UI systems comparison

   UI Toolkit, uGUI and IMGUI.

2. UI Toolkit

   UXML, USS and VisualElement.

3. Unity UI (uGUI)

   Canvas, RectTransform and events.

4. Input System

   Actions, bindings and devices.

5. Animation

   Animator, clips and state machines.

6. Audio

   AudioSource and mixers.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.