Just adding on to louisaaa01's answer, (this is still oversimplified) your internal mark determines your rank amongst those in your school, and then it gets matched up to the range of external marks achieved by your school using some distribution to determine your internal mark. It's then averaged with your actual external mark to provide your HSC mark. Essentially, to maximise your ATAR you have to beat the majority of your cohort internally and get you and your whole cohort to perform externally - it's why the joke 'I'll help you after Trials' exists and makes sense.
The whole system basically exists to increase the fairness of exams from different schools; schools are thus given motivation to not, say, make their exams piss easy. In the situation you describe, there are many possible outcomes, and they depend on two main variables; how well your cohort has done compared to you internally and how well your cohort has done externally. If your rank is high, and your cohort does well, your 75-80 should be pulled up. If your rank is low and your cohort is strong and you underperform, you're still likely to get pulled up and worst-case scenario you maintain your mark (this is why performing together is so important!). On the other hand, if you rank highly and your cohort underperforms as well as yourself, you get dragged down - similar to if you rank lower and you and your cohort underperform; it's likely the best-case scenario is you maintaining your mark or marginally getting dragged up.
Your ATAR is then determined from the scaled aggregate of your HSC marks, and compared to the rest of the state. Note that NESA doesn't calculate the ATAR, UAC does - the ATAR exists for entry into tertiary level study, and the whole point of internals is for a rank and aligned internal mark - NESA doesn't otherwise care about them in the calculation of your overall HSC mark. Therefore, it's quite erroneous to state that NESA would take into consideration anything internally and also that anything internal equals a high ATAR - remember also that internals are just half the ATAR.
Hope this makes sense!