Take your pick....
The big one (still going on?) was it was being spread by Mobile 5G masts (no idea why 4G masts are not spreading it?). As it is a virus, this kind of ignores basic, fundamental science (and reality).
Other versions are this is preparation for a "1 world government" (very big in the US I hear). The idea is that this is all about making us follow the rules unquestioningly.
As an additional one to the above, there is one that when we are all so worried about Covid-19, "they" will provide a "vaccine" that will (in reality) be a microchip that allows them to track us, and possibly control us. This latter one is very popular in the US as well, as it ties into some fundamentalist "end of days" Christians who believe that "chips" will be implanted into us all to control us.
Then of course, there is the "big pharma" one, about how this is all invented, JUST so that "big pharma" can ride to the rescue and make millions/billions/trillions etc.
Finally, there is the "this is all a hoax to scupper Trump's election/frustrate Brexit/stop Bolton winning the Champions' League"" (etc. - take your pick on whatever you would like to see blocked).
The issue I suspect is that while people have always had some... err... dubious... theories, until the days of the internet, they were not self-re-enforcing. Nowadays, you can go online, find a group (somewhere in the world) who agrees with your theory du jour, and it "proves" that only the people who "do their own research" (i.e. watch a video on youtube) actually know what's going on.
Strange days indeed....