I am curious how hibernate handles the following:
It loads objectA from the session, loads a list with objects (including objectA), the list is removed in the running environment, thus only keeping a reference to objectA. The objects from the list except objectA are garbage collected by java. How does this reflect to the hibernate cache? Does it keep the objects in the cache? Or is this something I should not worry about because it's transparent to me.

Hibernate keep them we do not garbage collect.
You can do session.evict() or session.clear()