Once you have an identified case, you trace who they have been in close contact with recently and start contacting/testing/isolating them and trace it further f

Once you have an identified case, you trace who they have been in close contact with recently and start contacting/testing/isolating them and trace it further from there E.g. In Switzerland this was done manually (by medical staff mostly i think) in the very beginning, but they gave up very quickly on it because of lack of resources.