So recently, governments have been using 'conspiracy theorist' to attack basically anyone who does not accept government narratives. If you don't believe in their good will and morality, you are a conspiracy theorist. In fact, a local paper once claimed that people who believe that politicians are corrupt, or that the media is pushing an agenda, are conspiracy theorists. So much for that.
It doesn't matter if it actually involves a conspiracy or not. For example, if you believe that 9/11 was an inside job, then you are a conspiracy theorist. However, if you believe that the Nord Stream sabotage was NOT an inside job, you are a conspiracy theorist. The only thing those two have in common is that you are not a conspiracy theorist if you say the US government did nothing wrong.
Regardless of the double standards, the problem is the conspiracies do happen. Now, what actually is a conspiracy theorist? Because there are a good number of totally whacked people out there, who believe in wildly implausible conspiracies.
My best way to explain it is as follows: people who exclusively or preferentially explain things with 'conspiracies' are conspiracy theorists.
Hollow Earth is actually an interesting one. Almost certainly not a probable thing, but it's at least a fun one.
Reptilians? No. But it wouldn't surprise me if there were some degree of alien influence over governments. IE, certain conditional requirements in order for them to be open to trading technology.
Covid-wise, there's an endless supply of semi-plausible gains that could be had by releasing a "mildly" deadly virus with other side effects, along with a vaccine that provided other controlled negative effects. Even more-so if those effects could be delayed. Just think of the Aschen from Stargate SG1.
I think the real conspiracy about flat earth is that anyone actually believes in that anymore. I think it's a deliberate hoax to make certain sorts look crazier and easier to dismiss.