is random sampling and random selection the same thing?
Random selection is how you draw the sample of people for your study from a population. Random assignment is how you assign the sample that you draw to different groups or treatments in your study. It is possible to have both random selection and assignment in a study.
No. Please see the earlier post.
no random sampling refers to the people you choose for a study. an ideal study would pick people with a randomizer so that it represents the population.
random assignment refers to the roles those people play in the study (maybe they are the ones who take the placebo, or they actually experience treatment)