Asset: "An Asset is a representation of any item you can use in your game or Project. An Asset may come from a file created outside of Unity, such as a 3D Model, an audio file, an image, or any of the other file types that Unity supports. There are also some Asset types that you can create in Unity, such as a ProBuilder Mesh, an Animator Controller, an Audio Mixer, or a Render Texture."
How Unity handles Assets:
"In all cases, your original source file is never modified by Unity, even though within Unity you can often choose between various ways to compress, modify, or otherwise process the Asset. The import process reads your source file, and creates a game-ready representation of your Asset internally, matching your chosen import settings. If you modify the import settings for an Asset, or make a change to the source file in the Asset folder, that causes Unity to re-import the Asset again to reflect your changes.
Note: Importing native 3D formats requires that the 3D modeling software be installed on the same computer as Unity. This is because Unity uses the 3D modeling software’s FBX Exporter plug-in to read the file. Alternatively, you can directly export as FBX from your application and save into the Projects folder."
Where to get Assets:
1. Unity's Asset Store (available in the Editor in the Asset Store window or online at https://assetstore.unity.com/): https://docs.unity3d.com/Manual/AssetStore.html
* Note: Standard Assets, available for free on the Asset Store by Unity Technologies, is a popular collection of assets, scripts, and example scenes that "can be used to kickstart your Unity learning or be used as the basis for your own projects." (It is often used in tutorials.)
2. Your personal media (videos, images, custom models, audio files, etc.)
3. Websites for free 3D models (there are many; here are several popular ones):
* Note: In the section above on how Unity handles assets, it is mentioned that Unity prefers FBX file types. However, Unity can read a variety of file types: https://docs.unity3d.com/Manual/3D-formats.html
* Note: Sometimes, 3D model downloads will come with texture files. These may or may not map correctly to the model within Unity.
* Note: Some 3D models will import as Prefabs, so you will have to right click on them in the Hierarchy window and select "Unpack Prefab" to edit their components or child objects. Sometimes, 3D model downloads will come with "Camera" or "Lamp" child objects, which you may want to delete after unpacking the prefab, as they will likely be unnecessary or distorting in your scene.
Importing assets:
"You can bring Assets created outside of Unity into your Unity Project by either exporting the file directly into the Assets folder under your Project, or copying it into that folder. For many common formats, you can save your source file directly into your Project’s Assets folder and Unity can read it. Unity also detects when you save new changes to the file and re-imports files as necessary.
[* You can also import an Asset within Unity by right clicking in the Project window and selecting "Import New Asset..." or clicking the "Assets" dropdown in the menu bar and selecting "Import New Assets..."]
You can use the Project window inside Unity to view the contents of your Assets folder. Therefore, if you save or copy a file to your Assets folder, Unity imports it, and it appears in your Project window.
You can create as many folders as you like and use them to organize your Assets. [* For example, you may want to create different folders for Animations, Images, Materials, Meshes/Models, Scenes, Scripts, Sounds, Textures, Videos, etc.]
The simplest way to safely move or rename your assets is to always do it from within Unity’s Project folder. This way, Unity automatically moves or renames the corresponding meta file."
Asset Import Settings
"Each type of Asset that Unity supports has a set of Import Settings, which affect how the Asset appears or behaves. To view an Asset’s import settings, select the Asset in the Project View. The import settings for this Asset will appear in the Inspector. The options that appear vary depending on the type of Asset selected.
For example, an image’s import settings allow you to choose whether Unity imports it as a Texture, a 2D sprite, or a normal map. The import settings for an FBX file allow you to adjust the scale, generate normals or lightmap coordinates, and split & trim animation clips defined in the file."