I know some students just use the lecture notes but in your opinion, is that a good idea if they're not truly understanding the content?
If I really want to understand the content, is it better to get the textbook and use it?
Also, what textbooks are used for Fundamentals, Chemistry 1 and Chemistry 2?
You sound like you're overthinking it. It depends on the kind of learner you are. You also have to be flexible at university. Sometimes you get great lecture notes from the lecturers, sometimes they are absolutely awful. This can vary between lecturers in the same subject. As with everything, your greatest understanding is going to come from exposure to the material in lots of different forms. But that also sucks up heaps of time. I use lecture notes the least, to be honest, in all my subjects - they are a guiding thing for me, most of the time. I use textbooks (not just the prescribed one), YouTube, Khan Academy, Google, academic papers, etc etc etc to try and get a well rounded understanding of a topic. But it isn't particularly time efficient.
Nobody can tell you if the lecture notes are going to be enough. The best starting point if you want to understand content at uni level is: pre read lecture notes, attend all lectures, take your own notes (before, during or after), attend all tutorials/workshops, do all prescribed questions. Those things will generally get you over the line in a pass/fail sense. To do well, and get a comprehensive understanding of everything, you will need to double or triple the amount of hours you put in. As a general rule, UniMelb Science subjects are content heavy. Fundamentals of Chemistry fits in almost the entire VCE 3/4 curriculum into 12 weeks AFAIK.