Are Covid test results true?

A friend of mine went to get a Covid test yesterday fully expecting to have a nurse administer the test. She was surprised to find that she was handed the test to do herself.

My question is how can you be sure that all the people that have tested themselves have done it correctly?

Surely there are thousands more cases than we realise because people have decided not to push the stick far enough up their nose and to the back of their throat to get an accurate reading?

Am I alone in this?

I can’t believe they can base results on people doing their own tests!

This is true, I know of cases where children as young as 10 are expected to swab themselves. Yet health professionals are expected to undertake training to do it correctly!
I do question the validity of the test result when this happens.