If you show me where you struggle, I can try to explain it.

Here just a summary:
Y scaling (yellow) only scales the visual indication of the yellow movement bars. No other usage.
Y scaling (blue) only scales the visual indication of the blue sleep pattern graph. No other usage.
Trigger level defines when the alarm gets set off. If the blue graph is over the trigger level, the alarm gets activated.

Now, the trigger level is connected to the blue graph (obviously). if you stretch the blue graph more, the trigger level also will be drawn higher. This means, it goes with the blue graph.
Changing the scaling of the blue graph does not change the sensitivity. Changing it does nothing but change the visualization. Because of that the trigger level will always have the same value.

Just have a look on the 2 attached screen shots. The first one has an Y scaling (blue) of 40, the 2nd one of 60.
On both the trigger level is on 23. As you can see, the alarm got set off when the blue sleep pattern graph crossed the trigger line.
On both screen shots, the trigger line has the same relative position to the blue sleep pattern graph.

Now, how to set it up properly:
Do a recording for a night, or use one of the already existing records.
Now adjust the Y scaling (blue), so it is most of the time in light or deep sleep. I would say, on the shown example 40 seems to be a good value, maybe 30 or 50 would work too. How ever more than 60 seems to be too much, as it indicates you have been awake very often. Unless you had a very rough night, it is not realistic.
After you set the blue graph scaling, you can set the trigger level on the next recording. I suggest it to have it on top or a bit above the light sleep area. The slightly gray background will help you for that set up.
Unless you change your bed or position of the phone, the scalings will not have to be changed anymore. Changing the trigger level should be enoughs for the fine adjustments.

Changing the Y scaling for the yellow bars is actually not needed anymore since we use the blue graph for triggering.