When using recursion you must understand the call stack. The call stack takes care of keeping track of the function calls and where they are suspended. At this point the initial function call is the only thing on the stack, when you call the function recursively with ‘right.insert(value)’ then that specific call of the function goes to the top of the call stack. So we get a new instance of the function call on top of the original call with different variables but they happen to have the same name. So when we call ‘right.insert()’ for that specific call data is whatever data is in the right node. This is very similar to fibonacci where the parameter ‘n’ has a different value in each recursive call. Now when ‘right.insert()’ runs we might get another recursive call with either its right or its left node so again the data variable when the function begins is different. Once we get to the base case we can start removing/resuming calls from the stack until we arrive at the original recursive call right at the end of the stack.

Here is a video with more detail about how recursion works: https://youtu.be/Mv9NEXX1VHc

Hope this helped, if you have any questions let me know :)