How to use a script from one tree in another?

I have three independent trees with own scenes and scripts.
I want to make it so that when entering the arena, the spawner starts its work and displays the timer on the third tree (interface).

изображение

изображение

изображение

What is your question? What have you tried and what doesnt work?

I’m new to coding. I tried use groups, singletons, export, node, but this doesn’t work

I heavily favor using a controller file that coordinates activities. In that controller file, instantiate your time_defence object and connect the time_defence signals to the controller file.

The time_defence object will have a signal that it emits when the player enters the Area2D, which the controller file handles. In that handler, instantiate the Spawnerobject and the Interface object.