Just a small tipp for those who are using 3.1 to follow this course.
GDScript has a keyword named
class_name which allows to register a global class name for a script.
I used this for the CombinationGenerator like this:
class_name CombinationGenerator extends Node static func generate_combination(length): var combination =  for number in range(length): randomize() combination.append(randi() % 10) return combination
which then allows me to just do
func generate_combination(): combination = CombinationGenerator.generate_combination(combination_length)
in Computer.gd, as it is globally accessible thanks to