How to load images/object at runtime and/or create assets server-side?

Hi, I'm developing an app (iOS and Android) which needs to download contents (assets) dynamically at runtime from a web server. Many of these assets should be managed and "created" directly on the server from specific users.

These assetes could be:

• images (used as background or as texture for a cube, for example)

• 3d object (generated/exported in standard formats directly from other program as "cinema 4d studio", etc.)

• unity assets (created with Unity editor)

I saw that "AssetBundle" is the only way to add contents at runtime but I have some doubts:

1. "assetbundles" requires "Unity assets" or can be used on images/object 3d "not processed" by Unity?
   

2. for images and 3d objects is there any other way to add it at runtime?

3. eventually, is there any server component (unity related) that I can use to process images/object to create Unity assets in a "total automatized" way (even using Unity editor or command line scripting with a specific licenze on the server)?

Thanks in advance!