think of p<0.001 as if you did the study 1000 times, you could...

think of p<0.001 as if you did the study 1000 times, you could expect the result to occur at most once in the 1000 due to factors other than the treatment ie natural person variation in immune health, antibody production, anti-cancer immunity, better diet, lower comordbidities

p<0.05 which is generally considered minimum requirement ie 1 in 20 chance of a random effect
Also the more things you measure, the same effect
one study with a 1000 outcomes could also expect 50 outcomes to be p<0.05 by random effects. You can adjust for this through stats but this principle is why trials must outline specific primary and secondary outcomes ahead of time and show why they are large enough to test a treatment effect