If you want to understand JSON, I recommend starting with this video I made: API JSON Tutorial (Video)
But if you only need to get data from Firebase, it’s as simple as using a cloud variable with the path to that value such as “CICPhjX7MIgg9FGH8Cp1vo52GQD3/phonebook/cousin/name” which would return “david”.
You can see an example of what that looks like here: I want to manage the user's account！ - #7 by tg.202212281f5yop
Also check out the Cloud Variables video that is part of the documentation: Variables - Thunkable Docs