Since people couldn't sit them, teachers were asked to submit predicted grades for students and rank them based on their mock exams, any in class tests, things like that. This was all submitted to the government who then moderated the results. Of course, the government decided it knew all these students better than the teachers that have taught them for two years and moderated the results based on previous averages for the school and on averages for all subject areas. So basically, if a school year was particularly high achieving compared to previous years and students were set to achieve results like AAA, they were moderated down to something like BCC because the school had never got good results like this before. Similarly, if a school had an amazing maths department but a terrible chemistry department, students who might have got an A* in maths would receive a C because the school's bad chemistry department brought down the average. Unsurprisingly, schools in poorer and more deprived areas have worse grades and as such those students were affected the most, meaning that many students who are from heavily deprived areas who had high predicted grades were moderated down, and many have lost the opportunity to go to the universities they applied to because of the government's algorithm moderating their grades down. I hope that makes sense but it's a bit complicated!
I'll also add that the government's rhetoric was that this was all about providing students opportunities to 'level up', but their algorithm basically meant those who went to private school didn't have their results moderated down so they're allowed places in the top universities whereas students from deprived areas who had the opportunity to be the first in their families to go to university lost out because of the moderation. It's basically a huge mess and, in a country like the UK that already has many class problems, has just exacerbated the gap between the working class and everyone else.