What’s the difference between the two?
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.
Random sampling (or random selection) is the process by which you pull from the population. If you use RS you can generalize your results to the population.
Random assignment is used (after random sampling from the population) in an experiment. You assign the subjects to either the control group or the experimental group. This allows for the “control” of confounding variables. The experimental group gets the Independent Variable; the control group gets a placebo or no treatment.