I'll try to explain.
You need to understand the logic of Construct3 and then any mechanics will be as easy for you as opening a can of beer!
-- You see event number 4, it has a green arrow, it means that this event is a trigger. So what does this particular event number 4 mean. When any object in the “Alli” family triggers the timer with the tag “attack” then the function will be called. It's a separate trigger for each object in the family.
-- Now event number 6, you have to be careful with this event, it doesn't have an arrow, which means that this event will be executed 60 times per second. So 60 times a second Construct3 will check all the objects of the “Alli” family and will see if they have reached the base or not and if this object is moving and if the object is moving and it has already reached the base, then we will stop it and at the next check this object will not participate and we will start a timer for it.
-- You need to imagine how Construct3 goes through all the events on the sheet that do not have a green arrow and check if the condition is met.