There is no test for the “delta variant” other than a visual identification with an electron microscope. They are just making the number attributed to the variant up, because there is no way that 21,000 individual cases were positively identified.
Delta variant testing is done using PCR I believe.
They only test small percentages of Covid positive samples for variant type.
They start with this: https://www.nature.com/articles/nmeth.3625 Improved since 2015 of course. Then they dump all the data which is basically crap into a computer and try to figure out the sequence. They can do it for RNA as well as DNA. It's Garbage In, Gold Out, or so they say.