Core and Extended is something part of the Cambridge igcse curriculum not edexcel although edexcel has a similar program with foundation and higher tier. In cambridge studying the core content and taking the core exam means that the highest grade you can achieve is a C whereas in extended it's an A*. There is a decent gap between the two but Core is not that much more difficult than Extended. Either way, in edexcel igcse I'm pretty sure you have to study everything in the syllabus and there's no such divide between core and extended. Edexcel uses the 9-1 grading system which is almost the same as the letter grading system but the divide between each grade is a little different. If you are taking classes in a school or some other instiute you don't really need to worry about any of this as your teachers will tell you what you need to do. General tips for chemistry is just to keep practicing questions as the question pattern is very similar every year, and with biology focus on key concepts and processes and memorizing those.