So Modern history is and Physics are interchangeable?
Kinda. There are some questionable dot points like the example I gave you, or the development of AC and DC and discussing the roles of Westinghouse vs Edison. But those questions aren't the majority- they're usually in the multiple choice or there's one question on the development of something. Most questions are either asking you to sub numbers into a formula or explain how a formula works when applying it to a real-life situation. So, there's barely any math, but you need to have a strong understanding of the formulae.
I haven't used a periodic table in ages. Is HSC really heavily focused on nuclear stuff?
Tbh, I barely use the periodic table. It's probably needed if there's a question on doping semiconductors but I've never used it. Physics doesn't really focus on nuclear stuff. If a school chooses to study Quanta to Quarks as an elective (there are other electives to choose from), you study particle physics and get into a bit of quantum physics (e.g. quarks, muons), a bit on nuclear fusion/fission, and then you look at applications of radioisotopes and the uses of particle accelerators but you only study how they work and its applications rather than go into the chemistry of specific elements. And then there's a syllabus dot point on the Manhattan Project.
Essentially, physics likes to look at how a specific concept works (on a general basis) and then apply it to real-life situations (e.g. medical, agricultural applications) and then they like to ask its impacts on society and environment (these kinds of questions are common since they like to ask you to discuss its uses).