Once you’ve established the population you care about and the interventions you want to compare, your next step is deciding how you’ll measure the differences between your interventions.
Are you interested in which intervention yields longer survival? Which causes the fewest hospitalizations? Which one is the safest? In other words, what are your definitions of success?
You do this by defining your outcomes.
Outcomes are the metrics you use to evaluate the treatment approaches in your question. They’re your yardstick for objectively saying which intervention is better.