How to create functions that paste command-output to prompt-input-string like the ++ctrl+t++ function and cd jump like ++alt+c++

[MikeLemo1]

I'm having a hard time for some reason creating my custom function to paste the string output from fzf into the
user-prompt-input-box of the prompt after fzf choise enter.
I know how to say enter a vim editor with the fzf output string path but now sure how the default ++ctrl+t++ function works with the paste to user prompt functionality.
what makes ++ctrl+t++ paste the fzf output to user-promp-input-box?