An AssetBundle is content that is stored separately from a main game or application and loaded/downloaded at runtime. This helps minimize the impact on network and system resources, allowing customers to download and install only the parts they need. AssetBundles can also be used to update or add content post-release.

to create them in Unity we must use an editor script that creates a new command. To do this we create a folder Editor and inside it, a script called CreateAssetBundle and insert the code that I leave below.

before experimenting with the new key we must go to create our AssetBundle we select the object we want to use, in my case the prefab of a GameObject.

at the bottom right we can see the writing AssetBundle to create a new one just select it and choose new to create a new one and assign a name.

Finally, we should go to try the new editor command created. The command at the end of its correct execution will create a folder with the file to upload online and recall when the time is right.

To better understand the use I refer you to the guide of Unity Learn

project manager by day, unity developer by night