Thanks for the feedback.Thanks for all the comments! The depots work so that you can have as many lines as you want using the same depot. The vehicles return to it after driving the route and receive maintenance. The depot sends out a vehicle whenever a timetable on a line attached to it demands an vehicle, as long as there are some available. The only limitation is a depot's maintenance capacity, the workers only have time to cater to a certain number of vehicles and their condition will deteriorate if there are too many, causing more breakdowns.
You could easily have one depot which services one line at day and the same vehicles are used on a different line by night.
Really appreciated that you are taking the time to give ys some answers in the forum.