i don’t believe its possible but I would love to be proven wrong.
how would you export an asset bundle from in game. for example a user uploads a model. build asset bundle and exports it
You cannot do this. Simply put, the creation of asset bundles is done by editor code not runtime code.
If you are just looking to allow user-generate content to be saved/shared, you can write the data to a file and upload it to a server. You can then retrieve the file from the server and read it back into the game. You do not need to use AssetBundles to load content into Unity.