It's a little bit of both, depending on what university, what subject, and then what each individual lecturer is like.
No lecturer will ever give you EVERYTHING that you need, only the bare basic bibliography with maybe a couple of specialist titles thrown in there for interest. Whilst it's possible to achieve a somewhat decent mark relying on this list, you'd need to go further and find your own material too. Not only does it show the lecturer that you're making the effort, but it also helps to consolidate everything that you've learnt.
The easiest ways of sourcing for me (I do Ancient History, but it's essentially the same for most subjects):
-Look at the basic bibliography and read through some of the material
-Look at the footnotes and bibliographies in the books themselves, hunt those references out, and read those as well (Mining for sources is a great way as well to see how different thoughts and theories have changed over time!)
-Look online (Your university will have a subscription to a massive journal database called JSTOR, as well as their own library e-book database. JSTOR will become the greatest thing in your life, I guarantee it.) There's no point in randomly searching words, because that will give you anything and everything, barely any of it useful to your essay most of the time. Have a look through your lecture notes, have a look at any little lecture blurbs or module handbooks, see what sort of topics and concepts it emphasises, and go from there.
-Remember as well, if you have a massive struggle finding books, GO TO YOUR LECTURER AND ASK!!!!! I can't stress this enough. So many people are scared of going to lecturers office hours for fear of being made to look like an idiot. This couldn't be further from the truth, a good 90% of lecturers will be more than willing to give you advice and guidance if you admit to them you need some help.
University essays and assignments are very different from A-Levels in terms of the degree of independent learning and the concepts that you cover, but this is why 1st year doesn't usually count towards your degree classification. It's meant to be a learning curve, and working out how to find this information out and apply it to your work. I wouldn't stress about it too much now, topics and information/advice lectures and seminars in the first week or so of term will give you an idea of the sorts of things they're looking for!
Hope that helps!