The above assertion is undoubtedly correct and should hopefully result in some relief for you as a year 11 student going into year 12. Your question can however be answered from the relevant perspective, i.e. the year 12 perspective, which is the year where every assessable component (i.e. assessment task/exam) you complete ultimately contributes towards your ATAR.
In year 12, your ATAR is affected by your internals. Your internals constitute a number of school-based assessment tasks/exams that you complete in your respective subjects, all of which will eventually result in a final rank for each of your subjects. Final ranks indicate where you placed relative to your cohort. For example, 2/50 in English Advanced means you came 2nd in English Advanced relative to your school cohort. Ranks are a priority with respect to internal performance as final ranks are used to determine your Assessment Mark (which contributes 50% towards your final mark for a subject) for each of your subjects, through the moderation process. For optimal performance, you must therefore ensure your ranks are as high as possible, which can only be done through favourable results in your school-based assessment tasks/exams. This also means that the higher your ranks, the better the Assessment Marks you can expect.
If your internal performance happens to (hopefully not) be of rather low standards, you will still have a chance of compensating to an extent, and hopefully achieving a good ATAR. This is done by performing to the best of your abilities in the HSC exams for your respective subjects. The HSC exams contribute 50% towards your final marks across your subjects and are based solely on your own performance, meaning that they are not affected by factors such as your own ranks or your school rank. Of course, you wouldn't want to put yourself in such a situation to begin with. A great start to your final year of school entails a determined mindset, meaning that you will be willing to put in the effort by regularly studying and staying up to date with relevant content across your subjects, allowing you to perform well in your assessment tasks/exams. If you are capable of demonstrating a strong commitment, your goal of 90% across your subjects will definitely be achievable.
Lastly, regarding the number of units you're currently taking, it is useful to consider whether you will be keeping 12 units as you commence year 12, or whether you will be dropping to 11 or 10 units? Taking 12 units means you have 2 backup units, which are beneficial in the event where unexpectedly low performance occurs in 2 of your units. Provided you have performed better in the rest of your units, the 2 units in which you didn't perform well will not end up counting towards your ATAR (except in the case of English, where the best 2 units of English always count towards your ATAR) and this removes the negative effect those 2 units would have otherwise had on your ATAR.
I hope this helps!
