To get an idea how this works, create the following then play around with it to get a better understanding of the formulas.
Make one Dropdown (Dropdown1 in this example), and set it's items to
[1,2,3,4,5,6,7,8,9,10]
Now create a 2nd dropdown (Dropdown2) and set it's items to
Switch(
Dropdown1.Selected.Value,
1, ["A","B","C"],
2, ["D","E","F"],
3, ["G","H","I"],
4, ["J","K","L"],
5, ["M","N","O"],
6, ["P","Q","R"],
7, ["S","T","U"],
8, ["V","W","X"],
9, ["Y","Z","AA"],
["AB","AC","AD"]
)
Now you have a working example of two dropdowns where the second chanegs based on the first