>How they took your sample? Blood or mucus from nose/throat?
That was blood test. Another type of test - PCR - is taken from throat, and detects virus in respiratory tract. It shows if you are contagious, but, as far as I know, has very large error rate. I decided to do only blood test, because it is too late for PCR for me (just my opinion). Even that antibody test may be flawed. Or they could just replace my blood sample with other in accident, or something like this - and you can't check it. Maybe doing multiple tests in different labs is a way, but I personally don't really care: I've had the symptoms, they are gone, whatever.
>Are immunoglobulins "one fits them all" type of reaction?
I don't know much about biology, but looks like igM is common, or at least very similar for many diseases, and igG is specific to single virus. So, igG is produced after some time, and igM are first ones, but less specific. Having only igM is not enough to detect anything except "looks like you are ill". Having igG says that you had that virus in past. Or it is error in testing, I guess some tests may mismatch antibodies, depending on disease and type of antibody.
>An acquaintance was ill last year, very ill, some upper respiratory disease, had severe immunreaction to that, they got scared it's maybe covid, they called ambulance. The ambulance did the test and found no covid present.