Downloading Asset Bundles

Hey guys, I'm trying to look into some options for updating my game that I haven't tried before. My next update is going to push the size over 20MB so I'm wondering if I can use in-app purchasing and asset bundles for the update.

My main question is the asset bundle. What are the limits? I have read that I can't have code in there. Can I simply have the script existing in the original (or in the next update) that can be used by things in the asset bundle? How would they attach. For example let's say there is a new enemy being added. It's behaviour script can't be in the asset bundle so how do I get it into the game?

Can they only be scenes? Or can they not be scenes at all? If your game is in one scene, how do I go about adding new things to it.

If I have the ability for the player to purchase new levels as a pack. How would I go about adding new menu items for these (which is in one scene) and then have code set up to manage loading levels that didn't exist before the purchase?

I hope I'm making sense here, just trying to wrap my head around the whole concept.

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