The reason why many people who contract COVID-19 and are asymptomatic is simple. It is because your immune system can kill the pathogen before you show symptoms. In other words, your body overwhelms the virus before it takes over larger areas of your body.
The answer to your second and third question is yes and no. Most people, that recover quickly without showing symptoms, signifies that they had a good immune system to fight it.
This is the general response to any pathogen that enters your body and is not distinct to COVID-19. Your immune system immediately starts fighting the pathogen off. And if your body is able to find the antibody fast enough, the pathogen will be immobilized and will be restrained from infecting more cells.
My source is these immune system videos:
(1) Immune System, Part 1: Crash Course A&P #45