The difference is one test detects: an enzyme; the other: a piece of one segment of a specific virus protein which needs to be magnified by hundreds of thousands to a billion, even a trillion times, to register on the test.
See the sense in that difference?
;-)
You have just captured the art of testing — you have to know what you are looking for, the best way to find it, and to time it correctly to make a test accurate — Its not just something that can be reduced to a bullet point on the internet.