Use the graph like a cut list
Total watch time can rise just because more people clicked a longer video. Retention tells you whether they stayed for the reason you promised. A flat-ish curve after a small early drop is healthy. A cliff means a specific moment failed.
Short form cares about completion and replays. Long form cares about average view duration and whether people reach the chapter you titled. Live cares about average viewers against your peak, and how many were still there after the start.