It's actually not that difficult to cast 200 flags each day. The problem is that most users usually don't spend that much time on Stack Overflow.
Your flag quota increases with the number of helpful flags up to the maximum of 100 flags on posts and 100 on comments.
Comment flags are the easiest to raise as there's plenty of comments that need to be removed. Of course, we do not want to flag every single comment. Whenever you see conversational comments (e.g. "thanks", "please wait, I will edit the post", "it's working now"), they need to be flagged as no longer needed. Either the system will automatically remove it or it will go to moderators who remove hundreds if not thousands of such comments each day.
Comments are easy to flag because you don't need to look hard for them. They are pretty much everywhere, especially on new questions with answers. You could also use a SEDE query to look for some easy to flag comments, but that is often considered a waste of time by many. If you can do it via a SQL query then you can write a piece of software to do it automatically for you.
Post flags are the second type of flag. These included spam, R/A, NAA, close and custom flags.
Spam and R/A are not easy to find because Charcoal and moderators take care of it very quickly. You can watch Smoke Detector in Charcoal and you might be able to cast a few of them.
Close flags can only be raised by users with less than 3k reputation
NAA (not an answer) and VLQ flags are for posts that aren't an answer in any kind of shape or form. For example "I am having the same problem", "thanks for all answers", "here are more details for my question", etc. You might be surprised to learn that despite all the warnings, people post hundreds of such not-an-answers each day. All of them need to be deleted, but these are a little more difficult. While there are keywords and common phrases, each post is different. You have to read an answer, even if just to look for keywords, to see if it is an answer or a comment/question that needs to be deleted.
You can find some of them when browsing casually, but to find them in larger quantities you have to watch new answers as they are posted (Charcoal has a nice Blaze page that helps with this), search for keywords, use 10k tools NATO page, or write some automated software to sieve and filter them out.Custom flags can be about any issue that moderators need to be aware of or need to handle (but the community can't). These are the most difficult to rack up as there needs to be an issue in the first place that you can report.
Stack Overflow doesn't offer a detailed flag statistic overview so it's not easy to calculate which flags you have raised of the most. I can offer some insight into what I have been flagging and my rough estimates.
I have raised ~42k post flags. Most of them are on NAAs. I have spent some time in SOBotics and other tools available on site. I also wrote my own bot to find them and auto flag the most obvious ones. This saves me a lot of time, but there are still plenty of those that need to be looked at by a human. Over 1.5k on spam or R/A content. I have also flagged hundreds of issues I saw such as voting fraud.
I raised 49k+ comment flags. Most of them using automated software. I remove "thanks", "edited", "worked" on older posts. I estimate that I raised close to 10k of them manually.
Some trivia:
The topmost flagger as of today has raised 253,000+ helpful flags
Moderator Bhargav Rao has handled 500,000+ flags
Users (including Community bot) have raised 1,200,000+ flags in the year 2020
