I have heard that once you are affected by covid 19 and survive it (remain alive) then your immunity system will not let you affect by the virus again ?
How true is that ?
It's completely true. It's basic biology. You get infected by a pathogen, your body builds up immunity through antibody production which recognizes virus particles and destroys them. The only case of someone getting "reinfected" by COVID-19 is if they had the disease once and get infected by a mutated form. Your body will repeat the cycle again producing antibodies against the mutated form.