Some small subset of COVID tests are sent to larger labs to perform a more detailed genetic test to determine specific variants. The prevalence of different variants within that set is extrapolated to the population of the region from which it originated.
If the lab finds that 87% of the samples from Adam's College are of the Omega Mu strain, then it's assumed 87% of total cases at Adam's College to be Omega Mu variant.
If the lab finds that 87% of the samples from Adam's College are of the Omega Mu strain, then it's assumed 87% of total cases at Adam's College to be Omega Mu variant.