I've got similar complaints of testing. One study I read regarding PCR testing said that if you have Covid and get tested on the day most likely to return a positive result, there is a 20% false negative rate.
My anecdote is that I tested 6 days after my wife had her most severe symptoms, and the beginning of my symptoms. A friend staying with us tested on the same day. We both got negative test results.
A week later I tested again and got a positive result. Similar story for our friend.
A positive result is likely very reliable, meaning there are few false positives. A person can test Covid positive and yet no longer be contagious somehow. The guidance is that 10 days after first symptoms of covid, and absent any worsening symptoms or fever, a person is no longer contagious. Not sure how that works since it took 2 weeks for me to lose taste, and 3 weeks to develop the most minor cough (like 3 coughs a day).
https://www.acpjournals.org/doi/10.7326/M20-1495
Anyhow, Musk has joined the ranks of the invulnerable, so we can resume life as normal.