1) basically the IBDP subjects are divided into 6 groups.
First is language and literature in which you choose which language you want to take as your first language. It can be language and literature or literature alone. You’d know the difference in the first day but basically lit only focuses more on reading books and doing tasks based on them while lang and lit focuses on books and articles.
Second is language acquisition in which you chose another language to be as your second language. It can be language B which means that you have background knowledge about it before or AB initio in which you are taking the language right from scratch. You can replace this with another first language subjects in which you’d earn the bilingual diploma.
Third is human sciences in which you choose subjects from business, economics, psychology, History and many more subjects depending on what your school offers in this group.
Fourth is the natural sciences in which you can choose from biology, chemistry, physics, design technology etc. In IB, the hardest science is supposedly physics because there is a lot of critical thinking
Fifth is mathematics in which you can choose further mathematics, math HL, SL or math studies. Math HL is the hardest subject in the IB
Sixth group is where you can choose an art like music, visual arts etc. OR you can choose another subject from the other 4 groups (excluding mathematics) instead of the art subject.
Now you can take any subject as an HL or SL except for math studies which is only SL, Environmental systems and societies which is only SL, AB initio languages which are only SL and further mathematics which is only HL. You have to take at least 3 HLs throughout your diploma. For the differences, personally the largest difference between HL and and SL subject is in the sciences and mathematics but more in mathematics. You’d find the subject specific differences in the guide or the teachers will tell you in the beginning of IBDP 1.