I cant access your file(s), or i am to dumb to figure out the link.
'Boolean variable (isLocket)' will pick ALL 'Door' s who's Boolean is set. So, you end up with a list of picked objects (with theire individual index (IID). I suppose most 'Door' s are just locked.
Now you ask your self: do i want to apply the actions to all of them
In the 'yes i do' case, there is no 'for each' needed.
In the other case you will have to reach out to each object one by one. Usually there will be a 'follow up' condition to see if each individual object meets some conditions. In your case 'when player overlaps this object'.
In general, this 'follow up condition' starts picking from the previous picked list (all those with the boolean set) and filters that previous picked list to meet the condition. (the result can be again a new list, depends on the condition)
When using a 'for each' this means each object, one by one. (the new list contains 1 object)