So when comes to building your basic_page when you set the input_file_var and output_file_var and give it a value of StringVar() you give it that value ( or in other words reset the saved data ) when you call the function again.
You were very close and all that has to be done is move the:
input_file_var = StringVar()output_file_var = StringVar()variables to place where they are called only once ( like the beginning ) so you can move them when you define the root objects like under the
options_frame = CTkFrame(app)And if you are confused about this or other aspects of the code you are using feel free to ask and me or someone here can explain :)