AssetBundles, Resources.Load, and "Best Practice" in Unity 2017

I was referred to the Unity documentation, which states that the Best Practice for Resources.Load is to not use it.
Instead, to use AssetBundles.

I went through the documentation several times, on multiple pages (tuts, best practice, api, etc.), and it was quite confusing, but not after I was convinced by Unity that AssetBundles are definitely the way to go. Nowhere can I get a clear or complete answer as to how & when assetbundles and their assets are loaded into memory. Not their...

AssetBundles, Resources.Load, and "Best Practice" in Unity 2017