This was actually a topic of discussion I had with a friend who is PhD, MD and was team lead of Immunology for Eli Lilly.
When you get COVID tested they do not know the actual strain you have unless it is cultured and tested. They don't do that except for a very small percentage of tests and usually only in a hospital setting.
It is PRESUMED that you have a certain strain, not confirmed. The presumption largely rests with computer models developed by the same numbskulls at Imperial College who are experts at creating scare-mongering "models" that predict all kinds of calamities that never happen.