When does an AssetBundle load it's contents?

If I load 10000 different asset bundles, would each bundle load it’s contents immediately, or is it deferred unti a resource inside the bundle, or the main asset itself is referenced?

Contents from an AssetBundle are not loaded into memory until one of the load functions is called from AssetBundle such as AssetBundle.Load. So, a more direct answer to your question is that the contents are not loaded immediately and instead loaded when AssetBundle.Load or similar is called.